More changes for initramfs and hw- targets with a base architecture. The $BASE_ARCH approach didn't work because targets need to override things like KERNEL_PATH, so they need to source the relevant details file themselves. Shuffle things around so they can do that: move $CROSS into cross-compiler.sh, add cross compiler to $PATH independently of that, remove $ARCH from CONFIG_DIR and change users to add it, have getconfig check $ARCH_NAME directory first, redo qemu_defaults to chop out initramfs options less uncleanly...

# Include base architecture

ARCH=mipsel
source "$CONFIG_DIR/$ARCH/details"

KERNEL_PATH=thingy