microblaze: Do not force saving variables to flash

There is no reason to save variables to flash only.
Select option via Kconfig instead.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/Kconfig

@@ -15,7 +15,6 @@ config TARGET_MICROBLAZE_GENERIC
 	select OF_CONTROL
 	select DM
 	select DM_SERIAL
-	select ENV_IS_IN_FLASH
 	select SYSRESET
 
 endchoice