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/usb_a9263_dataflash_defconfig b/configs/usb_a9263_dataflash_defconfig
index f138514..2a6c67d 100644
--- a/configs/usb_a9263_dataflash_defconfig
+++ b/configs/usb_a9263_dataflash_defconfig
@@ -17,6 +17,8 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 root=/dev/mtdblock1 mtdparts=mtdparts=atmel_nand:16m(kernel)ro,120m(root1),-(root2) rw rootfstype=jffs2"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="nboot 21000000 0"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "