MAKEALL: rename boards_by_* functions to targets_by_*

We expect boards_by_* function to return the 7th filed, 'Target',
not the 6th field, 'Board name'.

So the function names, boards_by_* are a little misleading,
and should be renamed to targets_by_*.

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