kmk: Add support for INST property to TASKS targets.

If set, it will cause an install step for the given target to the
specified location (according to normal INSTALLS rules).
kmk: Emit empty defines in CONFIG_FILES only if they are quoted.
kmk: Add CONFIG_DEFS and CONFIG_DEFS_D globals returning all config vars as defs.

The defs are in the form of DEF=VAL (or -DDEF=VAL for the _D version) to be used
in compiler calls etc.
kmk: Use double-colon for TASKS target shortcuts to avoid conflicts with kBuild.

See svn.netlabs.org/kbuild/ticket/115 for an explaination on why this conflict
happened.