core-image-tiny: Add receipe for new rootfs image type

Add receipe for for core-image-tiny ROOTFS type and related packagegroup
receipes: packagegroup-core-tiny-boot.bb and packagegroup-core-tiny.bb.notused

Update conf/layer.conf:
- Add "core" to the list of configured layers (BBFILE_COLLECTIONS)
- Define the parent directory of the OpenEmbedded Core Metadata layer
(COREBASE)
- Define the list of recipes that are completely stable and will never
change (SIGGEN_EXCLUDERECIPES_ABISAFE)

Signed-off-by: Paul Butler <paul.butler@windriver.com>
axm5500: adding support for new machine type

- Add new machine configuration file for axm5500
- Add new u-boot recipe for emulation
The new machine type (axm5500) provides support for building
u-boot for both the emulation and asic flavors. The u-boot.bin
and u-boot-spl.bin images for both emulation and asic versions
are generated.
- Update SRCREV for u-boot-lsi to tag 5.8.1.12
- Define compatible machines: axm5500sim, axm5500, elpaso (acp34xx)
- Update SRCREV for linux-yocto-custom-rt to 7.8.1.9
- Add standard and preemt-rt defconfig files for axm5500 machine

Signed-off-by: Paul Butler <paul.butler@windriver.com>