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/at91sam9n12ek_spiflash_defconfig b/configs/at91sam9n12ek_spiflash_defconfig
index 5a9b5c2..e704ee0 100644
--- a/configs/at91sam9n12ek_spiflash_defconfig
+++ b/configs/at91sam9n12ek_spiflash_defconfig
@@ -17,6 +17,8 @@
 CONFIG_SYS_LOAD_ADDR=0x22000000
 CONFIG_SPI_BOOT=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="setenv bootargs ${console} ${mtdparts} ${bootargs_nand};sf probe 0; sf read 0x22000000 0x100000 0x300000; bootm 0x22000000"
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y