Makefile: Include debug symbols in the .elf

Signed-off-by: Sergey Alirzaev <zl29ah@gmail.com>

src/pru/pru.lds

@@ -3,7 +3,9 @@ SECTIONS
 {
     /* binutils on the PRU doesn't support --gc-sections so manually
      * discard the .compile_time_request section */
+    /* and GDB extended debugging information */
     /DISCARD/ : {
         *( .compile_time_request )
+        *( .gnu.debug* )
     }
 }