stlink: add st-util page

pages/common/st-util.md

@@ -0,0 +1,15 @@
+# st-util
+
+> Run GDB server to interact with STM32 ARM Cortex microcontoller.
+
+- Run GDB server on port 4500:
+
+`st-util -p 4500`
+
+- Connect to GDB server:
+
+`(gdb) target extended-remote localhost:4500`
+
+- Write firmware to device:
+
+`(gdb) load {{firmware.elf}}`