buildroot

check-package: enable for boot/

This directory can benefit from this script to prevent common mistakes
when submitting patches.

In order to accomplish this:
Do not ignore anymore files from the boot/ directory.
Ignore boot/barebox/barebox.mk as it declares a package infra and not a
package itself.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>