imx6*.conf: Rework machine assignments for u-boot variables
The u-boot settings in imx6qdlsabresd, imx6qdlsabreauto, and imx6ulevk are
set to values that are correct for u-boot-fslc. Attempts to override these
settings to fix u-boot-imx builds have broken u-boot-fslc builds, and
vice versa. The u-boot-imx build is currently broken:
```
| install: cannot stat '/opt/work/upstream/fsl-xwayland/tmp/work/imx6qdlsabresd-fsl-linux-gnueabi/u-boot-imx/2020.04-r0/build/u-boot.img': No such file or directory
```
Rework the u-boot-fslc-specific overrides so they are used conditionally
only for u-boot-fslc. The default settings are correct for u-boot-imx.
Fixes: 880dbf2 ("fsl-base.inc: use conditional assignment for base variables") Fixes: e83f54d ("machines: imx-base.inc: fix failing u-boot builds") Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
