esp32/Makefile: Assign result of $call to dummy var for older make.

Make version 4.1 and lower does not allow $call as the main expression on a
line, so assign the result of the $call to a dummy variable.

Fixes issue #5426.

ports/esp32/Makefile

@@ -67,7 +67,7 @@ ESPIDF = $(IDF_PATH)
 else
 $(info The ESPIDF variable has not been set, please set it to the root of the esp-idf repository.)
 $(info See README.md for installation instructions.)
-$(call print_supported_git_hash)
+dummy := $(call print_supported_git_hash)
 $(error ESPIDF not set)
 endif
 endif
@@ -99,7 +99,7 @@ $(info The git hash of ESP IDF does not match the supported version)
 $(info The build may complete and the firmware may work but it is not guaranteed)
 $(info ESP IDF path:       $(ESPIDF))
 $(info Current git hash:   $(ESPIDF_CURHASH))
-$(call print_supported_git_hash)
+dummy := $(call print_supported_git_hash)
 endif
 
 # pretty format of ESP IDF version, used internally by the IDF