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

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython