imx-boot-container.bbclass: support having more than one UBOOT_CONFIG
The whole purpose of the UBOOT_CONFIG machinery is to allow one to
build multiple versions of U-Boot within the same recipe, e.g. one for
writing to an eMMC boot partition, and one for use with an SD card for
bootstrapping/recovery. But the current code here assumes UBOOT_CONFIG
consists of just a single word (and normally leading whitespace in that
variable would be harmless).
So use the variable ${type} which gets set in turn to each individual
word in UBOOT_CONFIG, which was probably the original intention
anyway. Add a comment about what the u-boot.itb and flash.bin symlinks
will end up pointing at.
When UBOOT_CONFIG does indeed consist of a single word only, this
changes nothing. But this unbreaks the build and makes the different
artifacts available for the UBOOT_CONFIG = "sd emmc ..." case that may
be used by custom boards.
