kernel-fitimage.bbclass: introduce FIT_SUPPORTED_INITRAMFS_FSTYPES

kernel-fitimage.bbclass: introduce FIT_SUPPORTED_INITRAMFS_FSTYPES

It was found when a end user wants to build a squashfs type initramfs
into fitimage, it just fails without printing out any error or warning
messages, which is not right.

Introduce a FIT_SUPPORTED_INITRAMFS_FSTYPES variable to avoid
hard-coding the supported initramfs types, and it could be overridden
in config files. Also break the build when none of a supported
initramfs type is found.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>