configs/pc: refactor to use genimage and grub.cfg

This simplifies the pc configs and respective post image scripts to use
the shared genimage script and separate grub config files.
Separate grub files are cleaner to maintain and easier to copy and
modify, for example to support booting the pc defconfigs in qemu.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>