MAKEALL: fix get_target_arch() to adjust to '-' in Status field

Since the Kconfig conversion, boards.cfg scanned by MAKEALL is
generated by tools/genboardscfg.py.  Every board is supposed to have
its own MAINTAINERS that contains maintainer and status information,
but, in fact, MAINTAINERS is missing from some boards.

For such boards, the first field, Status, is filled with '-'.
It causes a problem for "set" command, which ignores '-' in its
arguments.  Consequently, get_target_arch() returns a wrong field
and MAKEALL fails to get a correct toolchain.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>