MAKEALL: fix boards_by_field function

Commit 27af930e changed the boards.cfg format
and it changed boards_by_field() function incorrectly.
For tegra cpus it returned Board Name field,
not Target field.

This commit restores the behavior prior to 27af930e in the right way.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>