flash: declare local symbols as static

flash: declare local symbols as static

Functions and variables that are not used outside the file should
be declared as static.

Change-Id: I52d46ed6d4c9b98a7152eb23274c836416f409a3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5893
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>