fix(80usrgptprio): Add special x-initrd.mount option to /sysroot/usr

systemd-fstab-generator expects /usr to be in the root filesystem's
fstab rather than in the initrd's fstab as we do. This means the
filesystem target only winds up depending on /sysroot but not
/sysroot/usr. The special x-initrd.mount mount option forces the
filesystem target to depend on it as well.

Fixes issue #22
fix(80resize): Don't resize until after kexec
Merge pull request #23 from marineam/fix-usr

Fixes for resize in /usr images