imx6ullevk: Stop forcing the U-Boot and Linux kernel to imx flavour
We should avoid overriding the U-Boot and Linux kernel to imx flavour as
we might use mainline or their forks; this fixes build errors when using
fslc distros as:
,----[ Build error ]
| ERROR: Nothing PROVIDES 'virtual/kernel'
| linux-fslc-lts-4.19 PROVIDES virtual/kernel but was skipped: PREFERRED_PROVIDER_virtual/kernel set to linux-imx, not linux-fslc-lts-4.19
| linux-fslc-imx PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-qoriq PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-fslc PROVIDES virtual/kernel but was skipped: PREFERRED_PROVIDER_virtual/kernel set to linux-imx, not linux-fslc
| linux-imx PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-fslc-qoriq PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-yocto-rt PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-yocto-tiny PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-yocto-dev PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-yocto PROVIDES virtual/kernel but was skipped: incompatible with machine imx6ullevk (not in COMPATIBLE_MACHINE)
| linux-dummy PROVIDES virtual/kernel but was skipped: PREFERRED_PROVIDER_virtual/kernel set to linux-imx, not linux-dummy
| ERROR: Required build target 'core-image-minimal' has no buildable providers.
| Missing or unbuildable dependency chain was: ['core-image-minimal', 'virtual/kernel']
`----
Fixes: #432 Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
