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.

kerneltask/micropython,adafruit/circuitpython,tobbad/micropython,pozetroninc/micropython,kerneltask/micropython,henriknelson/micropython,selste/micropython,kerneltask/micropython,MrSurly/micropython,bvernoux/micropython,adafruit/circuitpython,henriknelson/micropython,pozetroninc/micropython,pramasoul/micropython,henriknelson/micropython,tobbad/micropython,tobbad/micropython,bvernoux/micropython,adafruit/circuitpython,henriknelson/micropython,bvernoux/micropython,selste/micropython,selste/micropython,pramasoul/micropython,pozetroninc/micropython,adafruit/circuitpython,MrSurly/micropython,MrSurly/micropython,kerneltask/micropython,MrSurly/micropython,bvernoux/micropython,henriknelson/micropython,pramasoul/micropython,kerneltask/micropython,selste/micropython,pramasoul/micropython,tobbad/micropython,selste/micropython,MrSurly/micropython,bvernoux/micropython,adafruit/circuitpython,pramasoul/micropython,pozetroninc/micropython,adafruit/circuitpython,pozetroninc/micropython,tobbad/micropython