kmk/kBuild: rename footer.kmk output variables like $(target)_OBJS_

[bird]

The variables set by footer.kmk in pass two will be named $(target)_2_BLAH. The variables set by subfooter.kmk and subheader.kmk or by footer.kmk in pass one will be named $(target)_1_BLAH. This will help avoiding confusion wrt to when they are safe to use and when second expansion is required.

Other variables:

• $(target)_OUTPUT_
• $(target)_OUTPUT_MAYBE_
• $(target)_DEPEND_
• $(target)_DEPORD_

Already official ones that may be considered for renaming to the new scheme:

• PATH_$(target)
• INSTARGET_$(target)
• TARGET_$(target)

The old values that are *known* to be in use will kick around until 0.3.0, starting with 0.2.1 kBuild will bitch about them.