check-package: enable for linux/

check-package: enable for linux/

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 linux/ directory.
Ignore missing LINUX_EXT_ prefix as the variables for linux extensions
do not use it.
Ignore this symbol:
 - LINUX_EXTENSIONS: defined by each linux extension, used by
   linux/linux.mk.

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