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_spiflash_defconfig b/configs/sama5d4ek_spiflash_defconfig
index e1f66a2..b36f4b5 100644
--- a/configs/sama5d4ek_spiflash_defconfig
+++ b/configs/sama5d4ek_spiflash_defconfig
@@ -32,6 +32,8 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 earlyprintk mtdparts=atmel_nand:256k(bootstrap)ro,768k(uboot)ro,256K(env_redundant),256k(env),512k(dtb),6M(kernel)ro,-(rootfs) rootfstype=ubifs ubi.mtd=6 root=ubi0:rootfs"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="sf probe 0; sf read 0x21000000 0x60000 0xc000; sf read 0x22000000 0x6c000 0x394000; bootz 0x22000000 - 0x21000000"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_SPI_LOAD=y