boot/grub: use install instead of cp

This way we don't depend on the permissions of files under boot/grub/,
which aren't tracked.

While we're at it, change all 'cp' usages into 'install' with the
correct mode for each file.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>