am335x_evm: Add am335x_boneblack variant

The BeagleBone Black differs from the other AM335x boards in a few
significant ways, so it makes sense to create a custom configuration
for it. In particular, it uses eMMC instead of NAND flash.

Signed-off-by: Justin Waters <justin.waters@timesys.com>
diff --git a/boards.cfg b/boards.cfg
index e49d6dd..211ed58 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -251,6 +251,7 @@
 am335x_evm_uart4             arm         armv7       am335x              ti             am33xx      am335x_evm:SERIAL5,CONS_INDEX=1,NAND
 am335x_evm_uart5             arm         armv7       am335x              ti             am33xx      am335x_evm:SERIAL6,CONS_INDEX=1,NAND
 am335x_evm_usbspl            arm         armv7       am335x              ti             am33xx      am335x_evm:SERIAL1,CONS_INDEX=1,NAND,SPL_USBETH_SUPPORT
+am335x_boneblack             arm         armv7       am335x              ti             am33xx      am335x_evm:SERIAL1,CONS_INDEX=1,EMMC_BOOT
 ti814x_evm                   arm         armv7       ti814x              ti             am33xx
 pcm051                       arm         armv7       pcm051              phytec         am33xx      pcm051
 sama5d3xek_mmc               arm         armv7       sama5d3xek          atmel          at91        sama5d3xek:SAMA5D3,SYS_USE_MMC