initscript: print runtime information at boot

This brings more context about the execution platform, and on what software (kernel + b2c version).

The expected output is this:

[0.73]: ## Runtime information ##

Linux version: 5.14.0-CI-00004-ga0f2a11480e6
Boot2container version: v0.9.2-3-g6a962cf-dirty
Architecture: x86_64
CPU: QEMU Virtual CPU version 2.5+
RAM: 358560 kB
Block devices: vda=128M