linux: Initial support for running Klipper in a Linux real-time process

Add support for compiling the Klipper micro-controller code as a
real-time process capable of running on standard Linux systems.

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

src/linux/Kconfig

@@ -0,0 +1,14 @@
+# Kconfig settings for compiling and running the micro-controller code
+# in a Linux process
+
+if MACH_LINUX
+
+config BOARD_DIRECTORY
+    string
+    default "linux"
+
+config CLOCK_FREQ
+    int
+    default 20000000
+
+endif