Boot: Fix GNUARM compile error

Initialise local variable to avoid GNUARM compile error
in case of MINSIZEREL build.

Change-Id: I0d06b79c6ee86f36e2668ac7a8b7ffff34ee2e2e
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/bl2/ext/mcuboot/bootutil/src/loader.c b/bl2/ext/mcuboot/bootutil/src/loader.c
index be0582e..406257e 100644
--- a/bl2/ext/mcuboot/bootutil/src/loader.c
+++ b/bl2/ext/mcuboot/bootutil/src/loader.c
@@ -1959,7 +1959,7 @@
 {
     size_t slot;
     struct boot_status bs;
-    int rc;
+    int rc = 0;
     int fa_id;
 
     /* The array of slot sectors are defined here (as opposed to file scope) so