Kconfig: Add new WANT_ADC option to reduce code size

Make it possible to not compile in support for ADC on chips with small
flash sizes.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/Makefile

@@ -3,7 +3,7 @@
 src-y += sched.c command.c basecmd.c debugcmds.c
 src-$(CONFIG_HAVE_GPIO) += initial_pins.c gpiocmds.c stepper.c endstop.c \
     trsync.c
-src-$(CONFIG_HAVE_GPIO_ADC) += adccmds.c
+src-$(CONFIG_WANT_ADC) += adccmds.c
 src-$(CONFIG_WANT_SPI) += spicmds.c
 src-$(CONFIG_WANT_I2C) += i2ccmds.c
 src-$(CONFIG_WANT_HARD_PWM) += pwmcmds.c