u-boot-imx-tools: Rework the COMPATIBLE_MACHINE strategy
We need to avoid setting the COMPATIBLE_MACHINE when not using an i.MX
based machine, otherwise we end with multiple provides and cause
problems for outside machines.
To support both use-cases, and allow the use of u-boot-imx-tools, we set
the recipe as invalid for all use, except for 'use-mainline-bsp' and
'use-nxp-bsp' based machines.
Refs: #573. Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Change-Id: I3b60828ce93c087a4774c71dbe62bab62bb6863d
