docs/cross-building.txt: add documentation about cross-builds

Cross-building IGT is not too trivial, as one needs to prepare
a sysroot directory containing not only glibc but also other
libraries. Document how to do it for IGT.

Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

docs/cross-building.txt

+Producing an IGT for another architecture
+=========================================
+
+Cross-build toolchain
+---------------------
+
+Producing cross-builds require installing a toolchain with support
+to the target architecture, or a toolchain built for the target,
+plus an emulator (like qemu). In the case of IGT, the minimal toolchain
+is GCC and binutils. For instance, on Fedora, to cross-build with arm64
+as the target architecture, those packages are needed:
+
+	binutils-aarch64-linux-gnu
+	gcc-aarch64-linux-gnu
+
+There are also tarballs with cross-compiler chains that can be used
+instead, like:
+
+	https://toolchains.bootlin.com/
+
+System root directory (sysroot)
+-------------------------------
+
+Besides a toolchain, a system root directory containing the libraries
+used by IGT pre-compiled to the target architecture is required.
+
+This can be obtained by either cross-building a distribution using
+Yocto, buildroot or similar, or by copying the system root from
+an existing installation for the desired architecture, containing
+the IGT needed build time and runtime library dependencies.
+
+Please notice that cross-build toolchains may require some
+dependent object files and libraries used by it to also be copied
+to the system root directory. For instance, in the case of Fedora,
+the files are located  under /usr/aarch64-linux-gnu/sys-root/
+(for aarch64 architecture) shall also be stored at the sysroot
+directory used by Meson, as otherwise the preparation step will fail.
+
+Meson preparation
+-----------------
+
+Meson requires an extra configuration file to be used for
+non-native builds. This is passed via --cross-file parameter to it.
+
+Such file contains details about the target OS/architecture, about
+the host (native) OS/architecture and declares the location of
+sysroot:
+
+- the [host_machine] section defines the system and architecture from
+  the native OS;
+  At the example below, the native OS is Linux x86_64 architecture.
+- the [target_machine] section contains details about the target
+  OS/architecture.
+  At the example below, the target is aarch64 (arm 64 bits architecture).
+- the [constants] section is optional, but it helps to use the sysroot
+  path directory on multiple keys at the [properties] section;
+- the [properties] section contains arguments to be used by the
+  the binaries;
+- the [binaries] section contains the binaries to be used during the
+  build. It can either be a native or a target toolchain.
+  If a target toolchan is used, an exe_wrapper key pointing to an arch
+  emulalor like qemu-arm is needed.
+
+The sysroot directory is where IGT dependent libraries and header
+files, compiled for a given architecture, are stored. At the example
+below, the sysroot is /aarch64-sysroot.
+
+Preparing for cross compilation is done by calling meson with the
+cross-compilation config file name, plus a build directory:
+
+	meson --cross-file arm64_cross.txt build
+
+The actual compilation can then be done using ninja:
+
+	ninja -C build
+
+Please notice that some parts of the IGT build are disabled during
+cross-compilation, like testlist file creation and documentation,
+as such steps depend on running the generated code at the native
+machine.
+
+The IGT root directory has already 3 examples that uses qemu to
+run a target-OS machine toolchain:
+
+	meson-cross-arm64.txt
+	meson-cross-armhf.txt
+	meson-cross-mips.txt
+
+The example below contains cross-build instructions when using
+a native cross-build toolchain.
+
+Example: arm64_cross.txt with a native cross-builder toolchain
+--------------------------------------------------------------
+
+[constants]
+sysroot = '/aarch64-sysroot'
+common_args = ['--sysroot=' + sysroot]
+
+[properties]
+sys_root = sysroot
+c_args = common_args
+c_link_args = common_args
+pkg_config_libdir = [sysroot + '/usr/lib64/pkgconfig', sysroot +'/usr/share/pkgconfig', sysroot +'/usr/local/lib/pkgconfig']
+
+[binaries]
+c = '/usr/bin/aarch64-linux-gnu-gcc'
+ar = '/usr/bin/aarch64-linux-gnu-gcc-ar'
+ld = '/usr/bin/aarch64-linux-gnu-ld'
+strip = '/usr/bin/aarch64-linux-gnu-strip'
+pkgconfig = 'pkg-config'
+
+[host_machine]
+system = 'linux'
+cpu_family = 'x86_64'
+cpu = 'x86_64'
+endian = 'little'
+
+[target_machine]
+system = 'linux'
+cpu_family = 'aarch64'
+cpu = 'aarch64'
+endian = 'little'