This properly exports the uImage kernel, which is used by u-boot, when
running 'flasher export'. Note the change to the uImage name to follow
the pattern "uImage-$flavor", which in the case of the RX 51 is
"uImage-nokia-rx51".
* allows to build/extract/list initramfs, add/del hook
* rebuild the initfs whenever running install or trying to flash/boot it
* flasher flash/boot: automatically set up a minimal rootfs with kernel and initfs,
if it does not exist yet