replace ifeq(xyz,) by ifndef(xyz)
set default goal to help target

If you use own targets in your .makerrc the first own target would be used as
default instead of the "help" target if you call `make PKG=packagename`.
This commit overwrite the default target and `make PKG=packagename` shows always
the "help" output.
separate PKG and PKGDIR; closes #13

This commit adds a new variable PKGDIR which replaces most use cases of PKG. PKG
is now used for building/naming the tarball and PKGDIR points to the source
directory (that contains the DESCRIPTION file).