genkernel: add simple udev support, include udev into initramfs if --udev
gen_compile: use LVM from system for the initramfs
gen_compile: use DMRAID from system, don't compile our own
gen_compile: use MDADM from system
linuxrc: don't try to be too smart wrt resolving REAL_ROOT symlink

This breaks LVM support with udev, don't do it, there is absolutely no reason
gen_initramfs: use portage-utils to determine the lvm2 udev rules files
gen_initramfs: differentiate between vital udev rules and non-vital