target: Switch to target_read_buffer() in verify_image fallback

target: Switch to target_read_buffer() in verify_image fallback

The current code checks the count to determine whether to read bytes or
words. However it fails to consider whether the base address is suitably
aligned.

Instead use the target_read_buffer() function which is for exactly this
purpose and generates optimal accesses with natural alignment.

Change-Id: I32ab5417890ee2219902df1529bc220fe353b4c7
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3217
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>