target: increase chunk size in dump_image

target: increase chunk size in dump_image

Replace the big stack-allocated buffer with a much bigger heap-allocated.
There was no explanation for the apparently arbitrary chunk size, and
performance was improved by increasing it, leveling out at about 4k.

Change-Id: I3b06d4469092ec8d89d0ce05bff0b7cf213c5062
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/404
Tested-by: jenkins
Reviewed-by: Marti Bolivar <mbolivar@leaflabs.com>
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>