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/ethernut5_defconfig b/configs/ethernut5_defconfig
index 1cf38b8..8c76a53 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -17,6 +17,8 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 root=/dev/mtdblock0 mtdparts=atmel_nand:-(root) rw rootfstype=jffs2"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="sf probe 0:0; sf read 0x22000000 0xc6000 0x294000; bootm 0x22000000"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTO_COMPLETE is not set