Improve support for archiso versioning

The version of the kernel contained in the archiso can now be set. The already
compiled packages must be contained in the directory.
build.sh: Improve match of linux version sed command

The sed command was matching and changing the spl version number along with the
linux version number. This caused makepkg to fail when building the zfs package
because it was searching for the incorrect spl version number.
Update to 0.6.2_3.12.2-1