Fix bugs in the blscfg module and in the 10_linux script for ppc64le
blscfg: Lookup default_kernelopts variable as fallback for options
Related: rhbz#1765297
10_linux.in: fix early exit due error when reading petitboot version
Resolves: rhbz#1827397
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>