kernel-fitimage: adding support for Initramfs bundle and u-boot script

kernel-fitimage: adding support for Initramfs bundle and u-boot script

This commit adds Initramfs bundle support to the FIT image in addition
to u-boot boot script capability.

These new features are selectable.

In case of Initramfs, the kernel is configured to be bundled with the rootfs
in the same binary (ie: zImage-initramfs-<machine>.bin). When the kernel is
copied to RAM and executed, it unpacks the Initramfs rootfs.

For more information about Initramfs please read:

https://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt

For more details about the Initramfs bundle and boot script implementation
please check the kernel-fitimage.bbclass paragraph in Yocto reference
or mega manual.

Current limitations:

- Initramfs bundle FIT support has been tested on ARM 32-bit
- The kernel image type in case of ARM 32-bit is zImage

Change-Id: I901bfd899e8d733c5b9a2b6645b1d4748f4b1fda
Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Backport of 19fa415c8769a67b52babd80f71d68bf36a21db2 in master branch
Signed-off-by: Remi Peuvergne <remi.peuvergne@non.se.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>