sys-power/bbswitch: 9999, 0.8-r2 - better kernel source dir handling,

sys-power/bbswitch: 9999, 0.8-r2 - better kernel source dir handling,

With approval of NP-Hardass adding myself to metadata. EAPI bump.
Fixing the way kernel sources are find, the hardcoded value uses running
kernel's /build dir, it was later altered to default /usr/src/linux.
This change makes it so it uses $KERNEL_DIR that defaults to
/usr/src/linux and fallback to running kernel's /build if no
/usr/src/linux exist.

Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: Piotr Karbowski <slashbeast@gentoo.org>