nor/flash: Add keep_alive() during flash write handler

nor/flash: Add keep_alive() during flash write handler

Added keep_alive() call inside main flash write loop. Large
files where causing a gdb timeout warning.

Change-Id: I525dad2d644e248dd9ecf678e4d8e33c22eefdf2
Signed-off-by: Edward Fewell <efewell@ti.com>
Reviewed-on: http://openocd.zylin.com/5682
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>