ARM: multi_v7_defconfig add /proc/config.gz

While 32-bit ARM isn't that widespread, multi_v7_defconfig is used for
building more generic images, where having access to config can be handy.

arm64 and x86_64 already bundle config.gz, so this is an effort to provide
the same level of comfort with ARMv7 devices.

Signed-off-by: David Heidelberg <david@ixit.cz>