boot: Fix uninitialized variable warning

Fix uninitialized variable warning that may occur when
compiler optimization is enabled (especially for size).

Change-Id: Id56669a2079ac20edbf74c17f943c974f1e8bf93
Signed-off-by: David Vincze <david.vincze@linaro.org>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 009f678..b5cbe28 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -1583,7 +1583,7 @@
 {
     size_t slot;
     struct boot_status bs;
-    int rc;
+    int rc = -1;
     int fa_id;
     int image_index;
     bool has_upgrade;