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/lib/cpus/aarch32/cpu_helpers.S b/lib/cpus/aarch32/cpu_helpers.S
index 900d158..d8cabfe 100644
--- a/lib/cpus/aarch32/cpu_helpers.S
+++ b/lib/cpus/aarch32/cpu_helpers.S
@@ -34,7 +34,7 @@
 #include <cpu_data.h>
 #include <cpu_macros.S>
 
-#if IMAGE_BL1 || IMAGE_BL32
+#if defined(IMAGE_BL1) || defined(IMAGE_BL32)
 	/*
 	 * The reset handler common to all platforms.  After a matching
 	 * cpu_ops structure entry is found, the correponding reset_handler
@@ -68,7 +68,7 @@
 
 #endif /* IMAGE_BL1 || IMAGE_BL32 */
 
-#if IMAGE_BL32 /* The power down core and cluster is needed only in  BL32 */
+#ifdef IMAGE_BL32 /* The power down core and cluster is needed only in  BL32 */
 	/*
 	 * void prepare_cpu_pwr_dwn(unsigned int power_level)
 	 *