tools/mkimage: fix handling long filenames

tools/mkimage: fix handling long filenames

The cmdline for calling the dtc was cut-off when using long filenames (e.g.
245 bytes) for output-file and datafile of "-f" parameter.
For FIT-images cmd[MKIMAGE_MAX_DTC_CMDLINE_LEN] is declared (hardcoded 512 bytes),
and contains some static values, the path of a tmpfile and a datafile. tmpfile is
max MKIMAGE_MAX_TMPFILE_LEN (256) and datafile might be also this size. Having two
very long pathname results in a truncation os the executed shell command, as the
truncated datafile path will not be found.
Redefine MKIMAGE_MAX_DTC_CMDLINE_LEN to "2 * MKIMAGE_MAX_TMPFILE_LEN + 35 for the
parameters.
This likely applies to the "-d" parameter, too.

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>