board: tec-ng: Do not make directories in a board Makefile

Commit e5c5301f refactored the build system not to make
directories in board makefiles.
But commit 8f380381 create directories again in
board/avionic-design/tec-ng/Makefile.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Alban Bedel <alban.bedel@avionic-design.de>