u-boot-imx-tools: Make recipe visible only for mx6, mx7 and mx8 machines
When building an image for a external meta-freescale machine,
e.g. raspberrypi3, and with meta-freescale in bblayer.conf we get:
NOTE: Multiple providers are available for u-boot-mkimage-native
(u-boot-imx-tools-native, u-boot-tools-native) Consider defining
a PREFERRED_PROVIDER entry to match u-boot-mkimage-native
Set COMPATIBLE_MACHINE to reduce visibility only for mx6, mx7 and
mx8 machines.
