board: add configuration for stm32f103c8 "Blue Pill"

The "Blue Pill" is a popular development board with an STM32F103C8
micro controller. According to sources, it has a 128kB Flash on board
even though the option bytes only report 64kB. This patch therefore also
modifies target/stm32f1x.cfg to take an optional FLASH_SIZE variable into
account which the board file sets to 0x20000.

Change-Id: I8a78ccd2b5faf637c539ee3cf8136789ee15c95d
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4495
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>