Convert CONFIG_USE_BOOTCOMMAND et al to Kconfig

This converts the following to Kconfig:
   CONFIG_USE_BOOTCOMMAND
   CONFIG_BOOTCOMMAND
   CONFIG_RAMBOOTCOMMAND
   CONFIG_NFSBOOTCOMMAND

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sama5d4ek_mmc_defconfig b/configs/sama5d4ek_mmc_defconfig
index ed5955d..b0689fb 100644
--- a/configs/sama5d4ek_mmc_defconfig
+++ b/configs/sama5d4ek_mmc_defconfig
@@ -30,6 +30,8 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p2 rw rootwait"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="if test ! -n ${dtb_name}; then setenv dtb_name at91-${board_name}.dtb; fi; fatload mmc 0:1 0x21000000 ${dtb_name}; fatload mmc 0:1 0x22000000 zImage; bootz 0x22000000 - 0x21000000"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set