gpiocmds: Add Kconfig option to allow boards to disable ADC commands

Some boards may not support the ADC hardware.  Update the build so
that those commands do not need to be compiled if they are not
available.

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

Makefile

@@ -25,8 +25,8 @@ CPP=cpp
 PYTHON=python
 
 # Source files
-src-y=sched.c command.c stepper.c basecmd.c gpiocmds.c spicmds.c endstop.c
-dirs-y=src
+src-y =
+dirs-y = src
 
 # Default compiler flags
 cc-option=$(shell if test -z "`$(1) $(2) -S -o /dev/null -xc /dev/null 2>&1`" \
@@ -58,6 +58,7 @@ MAKEFLAGS += --no-print-directory
 endif
 
 # Include board specific makefile
+include src/Makefile
 -include src/$(patsubst "%",%,$(CONFIG_BOARD_DIRECTORY))/Makefile
 
 ################ Common build rules