make: Add config for each ITE EC

EC_VARIANT is only used on AVR as a parameter to avrdude.

Replace its use on ITE with 2 choices:

- CONFIG_EC_ITE_IT8587E
- CONFIG_EC_ITE_IT5570E

Replace the EC defines with a matching define for the variable.

src/board/system76/lemp11/board.mk

@@ -4,7 +4,7 @@ board-y += board.c
 board-y += gpio.c
 
 EC=ite
-EC_VARIANT=it5570e
+CONFIG_EC_ITE_IT5570E=y
 
 # Enable eSPI
 CFLAGS+=-DEC_ESPI=1