Add blinky example for STM32F1 bluepill board (#1007)

Code taken from stm32f3_blinky example with LED port adapted to bluepill
board.
Port initialization and README from v1 rtic-examples with slight edits.

examples/stm32f1_bluepill_blinky/memory.x

@@ -0,0 +1,5 @@
+MEMORY
+{
+    FLASH : ORIGIN = 0x08000000, LENGTH = 64K
+    RAM : ORIGIN = 0x20000000, LENGTH = 20K
+}