stm32f1x_cl: Add initial support for STM32F1x connectivity line

This currently supports the stm32f107, but support will be added for
the F105 shortly (missing a linker script and I can't properly test
without a devkit).

cpu/arm/stm32f1x_cl/util.c

@@ -0,0 +1,12 @@
+#include <stdint.h>
+
+uint32_t retval;
+
+uint32_t
+get_msp(void)
+{
+  asm("ldr r1, =retval");
+  asm("mrs r0, msp");
+  asm("str r0, [r1]");
+  return retval;
+}