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/topic_miamiplus_defconfig b/configs/topic_miamiplus_defconfig
index dc03caa..bf36e51 100644
--- a/configs/topic_miamiplus_defconfig
+++ b/configs/topic_miamiplus_defconfig
@@ -20,6 +20,7 @@
 CONFIG_SYS_LDSCRIPT="arch/arm/mach-zynq/u-boot.lds"
 CONFIG_SYS_LOAD_ADDR=0x0
 CONFIG_BOOTDELAY=0
+CONFIG_BOOTCOMMAND="if mmcinfo; then if fatload mmc 0 0x1900000 ${bootscript}; then source 0x1900000; fi; fi; run $modeboot"
 CONFIG_USE_PREBOOT=y
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_SPI_LOAD=y