A variable called IMAGE_BOOTLOADER was being used without distinction for
referring to two different things:
- the recipe that builds the bootable artifacts
- the prefix of those artifacts
The value of this is "u-boot" for most platforms, but "imx-boot" for the
CC8X based platforms.
The name of the variable is misleading, so this commit splits it into two:
- BOOTLOADER_IMAGE_RECIPE, to refer to the recipe
- UBOOT_PREFIX, to refer to the prefix of the bootable artifact
With the separation, the variable UBOOT_SYMLINK becomes a generic formed
one, so it is moved to digi-defaults.inc.
While on it, fix the image_types_digi.bbclass which was not making use of
the original variable to establish all the dependencies.
Signed-off-by: Hector Palacios <hector.palacios@digi.com>