Rework incorrect use of assert() and panic() in codebase

Assert a valid security state using the macro sec_state_is_valid().
Replace assert() with panic() in those cases that might arise
because of runtime errors and not programming errors.
Replace panic() with assert() in those cases that might arise
because of programming errors.

Fixes ARM-software/tf-issues#96

Change-Id: I51e9ef0439fd5ff5e0edfef49050b69804bf14d5
diff --git a/include/common/bl_common.h b/include/common/bl_common.h
index e996fd6..9945e3a 100644
--- a/include/common/bl_common.h
+++ b/include/common/bl_common.h
@@ -33,6 +33,7 @@
 
 #define SECURE		0x0
 #define NON_SECURE	0x1
+#define sec_state_is_valid(s) (((s) == SECURE) || ((s) == NON_SECURE))
 
 #define UP	1
 #define DOWN	0