kmk: implement secondary target expansion

[bird]

GNU make sports a feature for secondary expansion of prerequisites, it will be extremely handy for kBuild to have a similar feature for targets (aka goals and files) as well. The kBuild use case is $(SomeTarget_PATH)/generated-file.h: prereq; $(APPEND) $@ whatever .

Modeling it after the secondary prerequisite expansion, the change consists of adding a new special variable .SECONDARYTARGETEXPANSION that is used to enable the feature. Detect targets needing secondary expansion in enter_file() when the string is in the cache. And finally do the expansion at the top of snap_deps().

[bird]

Implemented in [1701].