Reduce Helix keyboard build variation

The build commands for the Helix keyboard are:

make <helix_build_name>:<keymap_name>

For <helix_build_name>, specify the one in the rightmost column of the table below, such as helix, helix/pico.

before Oct 17 2019	Oct 17 2019	Mar 10 2020	Nov 29 2020
helix/rev1	helix/rev1	helix/rev1	helix/rev1
helix/pico	helix/pico	helix/pico	helix/pico
	helix/pico/back	helix/pico/back	helix/pico/back
	helix/pico/under	helix/pico/under	helix/pico/under
		helix/pico/sc	--
		helix/pico/sc/back	helix/pico/sc
		helix/pico/sc/under	--
helix/rev2 (=helix)	helix/rev2 (=helix)	helix/rev2 (=helix)	--
	helix/rev2/back	helix/rev2/back	--
	helix/rev2/back/oled	helix/rev2/back/oled	( --> helix/rev2/back)
	helix/rev2/oled	helix/rev2/oled	helix/rev2　(=helix)
	helix/rev2/oled/back	helix/rev2/oled/back	helix/rev2/back
	helix/rev2/oled/under	helix/rev2/oled/under	helix/rev2/under
		helix/rev2/sc	--
		helix/rev2/sc/back	--
		helix/rev2/sc/oled	--
		helix/rev2/sc/oledback	helix/rev2/sc
		helix/rev2/sc/oledunder	--
		helix/rev2/sc/under	--
	helix/rev2/under	helix/rev2/under	--
	helix/rev2/under/oled	helix/rev2/under/oled	( --> helix/rev2/under)