[entropy.spm] PortagePlugin: fix const_dir_readable() if condition
[entropy.spm] _add_kernel_dependency_to_pkg, rewrite to use /etc/kernels data

Trying to figure out the kernel owning /lib/modules/vermagic doesn't work
in all cases. Packages like sys-kernel/spl may be returned as valid
kernel packages. Now that /etc/kernels/vermagic consolidated, it's time
to use it.
Merge remote-tracking branch 'upstream/master'