linux: indicate proper CPE prefix

linux: indicate proper CPE prefix

The CPE type of the Linux kernel is special, it should be "o", unlike
all other packages that use "a". We therefore need to override
<pkg>_CPE_ID_PREFIX, so that the CPE ID of the linux package matches
with the CPE dictionary.

Reported-by: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>