board: toradex: colibri-imx(6ull|imx7): Fix missing fdt_fixup boot error

board: toradex: colibri-imx(6ull|imx7): Fix missing fdt_fixup boot error

In commit 51aaaf5e7975 ("board: toradex: imx: Remove not needed env variables")
the empty definition of fdt_fixup variable was removed, however this was
still referenced from the boot command leading to boot failures:
 ## Error: \"fdt_fixup\" not defined`

Fix this by removing "run fdt_fixup" from the boot command and instead
enable CONFIG_OF_ENV_SETUP in the defconfig that would achieve the same
but in a more robust way (it works fine even if the variable is not
defined).

Fixes: 51aaaf5e7975 ("board: toradex: imx: Remove not needed env variables")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>