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/pcm052_defconfig b/configs/pcm052_defconfig
index afdba34..83f7d93 100644
--- a/configs/pcm052_defconfig
+++ b/configs/pcm052_defconfig
@@ -15,6 +15,8 @@
 CONFIG_TARGET_PCM052=y
 CONFIG_SYS_LOAD_ADDR=0x82000000
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run bootcmd_nand"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y