boot: zephyr: remove [EXPERIMENTAL] from config option

Swap without scratch has been tested enough and no obvious bugs (or
"show stopper) bugs seem to exist; so remove [EXPERIMENTAL] and make
it "stable".

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index dd94696..0a8f359 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -127,7 +127,7 @@
 	  uses a much simpler code path.
 
 config BOOT_SWAP_USING_MOVE
-	bool "Swap mode that can run without a scratch partition [EXPERIMENTAL]"
+	bool "Swap mode that can run without a scratch partition"
 	default n
 	help
 	  If y, the swap upgrade is done in two steps, where first every