Kconfig: Hide DYNAMIC_CBMEM.

Only one setting actually works (exact value depends on board). So
no need to show it.

Change-Id: I2a85719264bbac07791ef6a9279590ba768c309e
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/7359
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins)

src/Kconfig

@@ -167,7 +167,7 @@ config BROKEN_CAR_MIGRATE
 	  CAR_GLOBAL and never do CAR migration.
 
 config DYNAMIC_CBMEM
-	bool "The CBMEM space is dynamically grown."
+	bool
 	default n
 	select EARLY_CBMEM_INIT
 	help