Use #ifdef for IMAGE_BL* instead of #if

One nasty part of ATF is some of boolean macros are always defined
as 1 or 0, and the rest of them are only defined under certain
conditions.

For the former group, "#if FOO" or "#if !FOO" must be used because
"#ifdef FOO" is always true.  (Options passed by $(call add_define,)
are the cases.)

For the latter, "#ifdef FOO" or "#ifndef FOO" should be used because
checking the value of an undefined macro is strange.

Here, IMAGE_BL* is handled by make_helpers/build_macro.mk like
follows:

  $(eval IMAGE := IMAGE_BL$(call uppercase,$(3)))

  $(OBJ): $(2)
          @echo "  CC      $$<"
          $$(Q)$$(CC) $$(TF_CFLAGS) $$(CFLAGS) -D$(IMAGE) -c $$< -o $$@

This means, IMAGE_BL* is defined when building the corresponding
image, but *undefined* for the other images.

So, IMAGE_BL* belongs to the latter group where we should use #ifdef
or #ifndef.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/include/common/bl_common.h b/include/common/bl_common.h
index 5076dfd..66c20fc 100644
--- a/include/common/bl_common.h
+++ b/include/common/bl_common.h
@@ -170,13 +170,13 @@
 extern uintptr_t __RO_END__;
 #endif
 
-#if IMAGE_BL2
+#if defined(IMAGE_BL2)
 extern uintptr_t __BL2_END__;
-#elif IMAGE_BL2U
+#elif defined(IMAGE_BL2U)
 extern uintptr_t __BL2U_END__;
-#elif IMAGE_BL31
+#elif defined(IMAGE_BL31)
 extern uintptr_t __BL31_END__;
-#elif IMAGE_BL32
+#elif defined(IMAGE_BL32)
 extern uintptr_t __BL32_END__;
 #endif /* IMAGE_BLX */