[PATCH master] firmware: fix dependency tracking for PBL firmware files

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

$(wildcard $(fwdir)/%) was added to allow for optional firmware, but in
return it broke dependency tracking, because it was evaluated before the
static pattern rule it's a part of.

Fix the dependency tracking by only evaluating it in a secondary
expansion. That this works as expected can be verified with:

  make -p | grep 'imx8mn-bl31.bin.gen.pbl.o:.*imx8mn-bl31.bin '

Which will return no result before this patch, but with it applied, it
will correctly show the dependency. Manual touching and rebuilding also
builds the firmware now as expected:

  AS [P]  firmware/imx8mm-bl31.bin.gen.pbl.o

Fixes: 5b171b7b7eda ("firmware: turn missing firmware into linker error")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 firmware/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/firmware/Makefile b/firmware/Makefile
index 5853950dfe4e..4fca83f808f6 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -99,7 +99,8 @@ clean-files += *.sha.bin *.sum
 $(patsubst %.gen.o,$(obj)/%.gen.o, $(obj-pbl-y)): $(obj)/%.gen.o: $(fwdir)/%
 
 # The same for pbl:
-$(patsubst %.gen.o,$(obj)/%.gen.pbl.o, $(obj-pbl-y) $(pbl-y)): $(obj)/%.gen.pbl.o: $(wildcard $(fwdir)/%)
+.SECONDEXPANSION:
+$(patsubst %.gen.o,$(obj)/%.gen.pbl.o, $(obj-pbl-y) $(pbl-y)): $(obj)/%.gen.pbl.o: $$(wildcard $(fwdir)/%)
 $(patsubst %.gen.o,$(obj)/%.extgen.pbl.o, $(pbl-y)): $(obj)/%.extgen.pbl.o: $(fwdir)/%
 
 pbl-y += $(addsuffix .extgen.o, $(fw-external-y))
-- 
2.39.2