refpolicy: add ability to specify policy version

refpolicy: add ability to specify policy version

Refpolicy by default will build the highest version supported. This may
cause older kernels to not load the policy.

This patch adds a custom policy version string which is defaulted to 30,
which is the highest supported as of today.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Acked-by: Matt Weber  <matthew.weber@rockwellcollins.com>
[Thomas:
 - rename option to BR2_PACKAGE_REFPOLICY_POLICY_VERSION
 - use qstrip to remove double quotes]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>