tools: Fix broken object compilation rules

As these rules depend on non-existing headers as well (likely copy &
pasted from fiptool), they never matched, and the built-in rules were
used. That led to random breakages when e.g. CPPFLAGS was suddenly
evaluated and contained invalid options.

For the stm32image, this reveals that we were relying on the built-in
rules by passing -D_GNU_SOURCE via CPPFLAGS, rather than using CFLAGS as
used in the local rule. Fix that as well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
diff --git a/tools/doimage/Makefile b/tools/doimage/Makefile
index bc74369..9cec681 100644
--- a/tools/doimage/Makefile
+++ b/tools/doimage/Makefile
@@ -39,7 +39,7 @@
 	@echo "Built $@ successfully"
 	@echo
 
-%.o: %.c %.h Makefile
+%.o: %.c Makefile
 	@echo "  CC      $<"
 	${Q}${CC} -c ${CFLAGS} ${INCLUDE_PATHS} $< -o $@