Add generic machinery to dynamically create files in $(TOPDIR). The
need arises because version controlled configuration files for
linters are going to be introduced.
For that, this commit introduces a variable $(TD_GENERATE_FILES),
which target all depends on, and which topdir.clean removes.
It defaults to another variable also introduced by this commit,
$(TD_COPY_FILES), which in turn defaults to $(TOPDIR)/conf/topdir.
This commit also adds support for JW_PKG_TOPDIR_COPY_PATH. It
supports a PATH-style syntax, which allows pointing to multiple
directories to be checked for source files. If they exist, they will
be appended to the files found in $(TOPDIR)/conf/topdir after
copying. Defining arbitray files to copy is not supported before
security implications during CI runs are better understood.
Having the copy prerequisites work comes at the cost of having to add
.SECONDEXPANSION. Since it's limited to the toplevel Makefile, I
suppose that's acceptable.
Signed-off-by: Jan Lindemann <jan@janware.com>
