debug: gdbstub: introduce gdb_bin2hex()

Adds a new function gdb_bin2hex() to convert binary into
hexadecimal string representation. This is similar to
bin2hex() but does not force a null character at the end
of the output buffer. This avoids an issue where the last
character of the hexadecimal string is replaced with
null character before sending to GDB.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>