[nrf fromtree] mcumgr/img_mgmt: Fix CONFIG_MCUBOOT_BOOTLOADER_NO_DOWNGRADE

Usage of the Kconfig, in code, has been missing CONFIG_,
so selected or not it did nothing.

(cherry picked from commit 1ec7116)
Signed-off-by: Dominik Ermel <[email protected]>

Author: de-nordic

subsys/mgmt/mcumgr/grp/os_mgmt/src/os_mgmt.c

@@ -469,7 +469,7 @@ os_mgmt_bootloader_info(struct smp_streamer *ctxt)
 
 		ok = zcbor_tstr_put_lit(zse, "mode") &&
 		     zcbor_int32_put(zse, BOOTLOADER_MODE);
-#if IS_ENABLED(MCUBOOT_BOOTLOADER_NO_DOWNGRADE)
+#if IS_ENABLED(CONFIG_MCUBOOT_BOOTLOADER_NO_DOWNGRADE)
 		ok = zcbor_tstr_put_lit(zse, "no-downgrade") &&
 		     zcbor_bool_encode(zse, true);
 #endif