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/pic32mzdask_defconfig b/configs/pic32mzdask_defconfig
index 4e86f32..ff6be78 100644
--- a/configs/pic32mzdask_defconfig
+++ b/configs/pic32mzdask_defconfig
@@ -13,6 +13,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SYS_LOAD_ADDR=0x88500000
 CONFIG_BOOTDELAY=5
+CONFIG_BOOTCOMMAND="run distro_bootcmd || run legacy_bootcmd"
 CONFIG_SYS_PROMPT="dask # "
 # CONFIG_CMD_SAVEENV is not set
 CONFIG_LOOPW=y