sysutils/u-boot-beaglebone: Fix eMMC boot

Due to rename of CONFIG_NAND -> CONFIG_MTD_RAW_NAND the BBB will not boot from eMMC.

https://github.com/u-boot/u-boot/commit/88718be3001055fa2801a44ab10570279b3f2cb7

https://github.com/u-boot/u-boot/commit/d4c2f6b7e3e2bfd297367d292aa641095c7a18e2

Submitted by: Oskar Holmund (oskar.holmlund@ohdata.se)
Differential Revision: https://reviews.freebsd.org/D25190