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/at91sam9g20ek_2mmc_defconfig b/configs/at91sam9g20ek_2mmc_defconfig
index 33d5e59..97057b5 100644
--- a/configs/at91sam9g20ek_2mmc_defconfig
+++ b/configs/at91sam9g20ek_2mmc_defconfig
@@ -20,6 +20,8 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 earlyprintk mtdparts=atmel_nand:256k(bootstrap)ro,512k(uboot)ro,256k(env),256k(env_redundant),256k(spare),512k(dtb),6M(kernel)ro,-(rootfs) root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="fatload mmc 0:1 0x22000000 uImage; bootm"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="U-Boot> "
 # CONFIG_CMD_BDI is not set