Convert CONFIG_BOOTARGS to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTARGS

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sama5d2_icp_mmc_defconfig b/configs/sama5d2_icp_mmc_defconfig
index dc4e815..8f3efd8 100644
--- a/configs/sama5d2_icp_mmc_defconfig
+++ b/configs/sama5d2_icp_mmc_defconfig
@@ -25,6 +25,7 @@
 CONFIG_SD_BOOT=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p2 rw rootwait"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SPL_SEPARATE_BSS=y
 CONFIG_SPL_DISPLAY_PRINT=y