resin-u-boot.bbclass: Make RESIN_UBOOT_MMC_DEVICES generic
resin-u-boot.bbclass: Make RESIN_UBOOT_MMC_DEVICES generic

We rename RESIN_UBOOT_MMC_DEVICES to RESIN_UBOOT_DEVICES because we now
also handle checking for flasher images on the USB media.

Signed-off-by: Florin Sarbu <3a837a37a2ac9072a088641ec330e0e5d5b86f4e@resin.io>

resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin,resin-os/meta-resin