conf: dra7xx-evm: avoid missing dtb files when ARAGO_BRAND=mainline

For ARAGO_BRAND=mainline with the dra7xx-evm MACHINE, avoid the
inclusion of device tree files that do not exist upstream.

Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Signed-off-by: Ryan Eatmon <reatmon@ti.com>

conf/machine/dra7xx-evm.conf

@@ -12,7 +12,7 @@ KERNEL_DEVICETREE = " \
     dra7-evm.dtb \
     ti/dra7x-evm-osd-lcd-common.dtbo \
     dra71-evm.dtb \
-    dra71-evm-nand.dtb \
+    ${@oe.utils.conditional('ARAGO_BRAND', 'mainline', '', 'dra71-evm-nand.dtb', d)} \
     ti/dra71-evm-lcd-auo-g101evn01.0.dtbo \
     dra72-evm.dtb \
     dra72-evm-revc.dtb \