nor/kinetis: add keep-alive during flash write

nor/kinetis: add keep-alive during flash write

Flashing Kinetis MCU MK22FX512VLH12 takes longer than one second,
thus gdb connection can timeout. Before commit 7f260f5009a7
("helper/command: Handle Tcl return values consistently") the
openocd timeout warning was masked by a bug in gdb_put_packet()
that resets the timeout counter if any message is logged out.
The commit above removes one log message and the timeout warning
is now triggered.

While the bug in gdb_put_packet() is still to be fixed, the reason
of the timeout is the lack of keep_alive() calls in kinetis flash
driver.

Add keep_alive() call at every iteration in function
kinetis_write_sections(), that is reported as core write function
in chunks of 1024 bytes in the log from the reporter.
Add also a keep_alive() call at every iteration in function
kinetis_write_inner(). This part is not present in the log but, by
code analysis, it represents another critical loop.

Change-Id: I38e631b36c7eb2f1e21cef68710ce47c03d3edda
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Jonatan Hatakeyama Zeidler <jonatan.zeidler@medineering.de>
Reviewed-on: http://openocd.zylin.com/5703
Tested-by: jenkins
Reviewed-by: <jonatan_zeidler@gmx.de>