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/slimbootloader_defconfig b/configs/slimbootloader_defconfig
index 5cfbaa2..fb2091b 100644
--- a/configs/slimbootloader_defconfig
+++ b/configs/slimbootloader_defconfig
@@ -7,6 +7,8 @@
 CONFIG_BOOTSTAGE=y
 CONFIG_BOOTSTAGE_REPORT=y
 CONFIG_BOOTDELAY=10
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="if test ${bootdev} = 'usb'; then ${bootdev} start; fi; if test ${bootdev} = 'scsi'; then ${bootdev} scan; fi; ${bootdev} info; ${bootfsload} ${bootdev} ${bootdevnum}:${bootdevpart} ${loadaddr} ${bootfile}; ${bootfsload} ${bootdev} ${bootdevnum}:${bootdevpart} ${ramdiskaddr} ${ramdiskfile}; zboot ${loadaddr} 0 ${ramdiskaddr} ${filesize}"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_BOARD_EARLY_INIT_R=y
 CONFIG_LAST_STAGE_INIT=y