board/pc: ensure grub.cfg is copied to target filesystem

board/pc: ensure grub.cfg is copied to target filesystem

Before this commit, the grub configuration file was copied to the
TARGET_DIR in a post-image hook, after the filesystem has been
generated. It was kinda working because the board/pc's grub
configuration and the default one are the same and the later was
copied during the build process of the grub2 package.

This commit ensures the custom board/pc grub configuration is copied at
the right time.

Signed-off-by: Grégoire Delattre <gregoire.delattre@gmail.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>