Fix overflow of filename

Fix overflow of filename

In thor protocol, only filename with 32 characters is allowed to
send but there was overflow of filename more than 32 and it is
crashed with 193 or longer length filename. Fix the overflow of
filename.

Note: ARTIK530 and ARTIK710 already misuse overflowed 33 length dtb
filename, and exact modification as thor protocol breaks flashing
on the boards. So fix with extention for the ARTIK borads by
appending more 32 bytes in rqt_pkt.str_data[1] as artik u-boot
expects.

Change-Id: I4081024cafa276617497091e00cbb6fbc21348b7
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>