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/socfpga_stratix10_defconfig b/configs/socfpga_stratix10_defconfig
index 21014f9..16ae0fe 100644
--- a/configs/socfpga_stratix10_defconfig
+++ b/configs/socfpga_stratix10_defconfig
@@ -12,6 +12,8 @@
 CONFIG_SPL_FS_FAT=y
 CONFIG_SPL_TEXT_BASE=0xFFE00000
 CONFIG_BOOTDELAY=5
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="earlycon"
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="SOCFPGA_STRATIX10 # "