meta-agl-core: remove IMAGE_FSTYPES override

meta-agl-core: remove IMAGE_FSTYPES override

Remove the anonymous python logic added to override any changes
to IMAGE_FSTYPES via _append/_remove by BSPs to fix SPEC-3741.
The updated Renesas BSP no longer does so, and removing this
logic avoids issues when using meta-agl-core with distribution
configurations other than poky-agl.

Additionally:
- Change the IMAGE_FSTYPES and INITRAMFS_FSTYPES assignments in
  poky-agl.conf back to regular ("=") instead of immediate
  (":=") so that things like the _appends being done to
  AGL_DEFAULT_IMAGE_FSTYPES for netboot and CI usecases will
  take affect. They were only currently working because of the
  later evaluation in the anonymous python logic that is being
  removed.
- Rationalize agl-image-minimal.inc to just include
  agl-image-boot.inc, and remove the unnecessary use of the
  AGL_EXTRA_IMAGE_FSTYPES variable in agl-image-boot.inc.

Bug-AGL: SPEC-3897

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Id655683c705678dd3ab1caff8980dba06f46a1bc
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26292
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account