spicmds: Add Kconfig option to allow boards to disable SPI commands

Some boards may not support SPI transfers.  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>

src/Kconfig

@@ -18,6 +18,9 @@ source "src/simulator/Kconfig"
 config HAVE_GPIO_ADC
     bool
     default n
+config HAVE_GPIO_SPI
+    bool
+    default n
 
 config INLINE_STEPPER_HACK
     # Enables gcc to inline stepper_event() into the main timer irq handler

src/Makefile

@@ -1,4 +1,5 @@
 # Main code build rules
 
-src-y += sched.c command.c stepper.c basecmd.c gpiocmds.c spicmds.c endstop.c
+src-y += sched.c command.c stepper.c basecmd.c gpiocmds.c endstop.c
 src-$(CONFIG_HAVE_GPIO_ADC) += adccmds.c
+src-$(CONFIG_HAVE_GPIO_SPI) += spicmds.c

src/avr/Kconfig

@@ -6,6 +6,7 @@ config AVR_SELECT
     bool
     default y
     select HAVE_GPIO_ADC
+    select HAVE_GPIO_SPI
 
 config BOARD_DIRECTORY
     string