base: kernel-lmp-fitimage: add support for FIT_LOADABLES

[mike-scott]

Extend this class to handle FIT_LOADABLES as a list of deployed files
to be added to the kernel fitimage. This allows subscribers to
retreive these files in u-boot where the contents are verified prior
to using.

Example would be to add the following line to machine.conf:
FIT_LOADABLES = "core0_m4_image.bin core1_m4_image.bin"

• A recipe would place those files in ${DEPLOYDIR} in do_deploy
• Then an additional do_compile[depends] is added to the kernel
  recipe such as:
  do_compile[depends] += ":do_deploy"

[mike-scott]

I might need to add to the commit description or split into 2 patches because there are several things happening here:

1. added a param to fitimage_emit_section_config which is the conf name. This is because it won't always be a DTB name.
2. added a 2nd param to fitimage_emit_section_config which is the loadable ID.
3. added fitimage_emit_section_loadable

[mike-scott]

Split this into 2 patches showing the adjustments to this code in a better way

[mike-scott]

Adjusted per discussion with @ricardosalveti (back to 1 commit)

[mike-scott]

@ricardosalveti This is ready for review. I've tested it.

[ricardosalveti]

Great, looks good, thanks!