Fix debug prints when loading to flash

Fix debug prints when loading to flash

While loading to flash with debug level at least 3,
OpenOCD tries to print the whole loaded bitstream.
This will be very-very-slow due to implementation of
conversion from buffer to string.

* fix condition on selected debug level in jtag/core.c
* replace slow buf_to_str function from helper/binarybuffer.c
  with faster but_to_hex_str function

Change-Id: I3dc01d5846941ca80736f2ed12e3a54114d2b6dd
Signed-off-by: Samuel Obuch <sobuch@codasip.com>
Reviewed-on: http://openocd.zylin.com/5800
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>