ramdisk: switch from initrd (ext4 based) to initramfs (cpio.gz based) format

ramdisk: switch from initrd (ext4 based) to initramfs (cpio.gz based) format

Initrd-style booting is deprecated since Linux kernel commit 9acc17baf1fd
("initrd: mark initrd support as deprecated") included in v5.9 release.

Since commit ff26956875c2 ("brd: remove support for BLKFLSBUF") included
in Linux kernel v4.10 it is no longer possible to free the memory
occupied by the initrd.

Switch Tizen ramdisk images to initramfs format (cpio.gz based) to comply
with the new approach of starting Linux based systems.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: Ie2658e0a445ea4c4a55d53e3b6e51c0c38d69496