arm: socfpga: misc: Export bootmode into environment variable

setenv an environment variable called "bootmode" , which contains the
board boot mode. This can be in turn used in scripts to determine from
where to load kernel and such.

Signed-off-by: Marek Vasut <marex@denx.de>

include/configs/socfpga_common.h

@@ -18,6 +18,7 @@
  */
 #define CONFIG_DISPLAY_CPUINFO
 #define CONFIG_DISPLAY_BOARDINFO_LATE
+#define CONFIG_ARCH_MISC_INIT
 #define CONFIG_ARCH_EARLY_INIT_R
 #define CONFIG_SYS_NO_FLASH
 #define CONFIG_CLOCKS