pkg-virtual.mk: explicitly set <pkg>_VERSION / _SOURCE for robustness

pkg-virtual.mk: explicitly set <pkg>_VERSION / _SOURCE for robustness

Recently a build failure was reported which was traced back to to the fact
that the user had a TOOLCHAIN_VERSION environment variable set which leads
to a strange looking error message:

toolchain/toolchain/toolchain.mk:40: *** TOOLCHAIN_SITE cannot be empty when
TOOLCHAIN_SOURCE is not.  Stop.

Environment variables automatically gets converted to make variables by GNU
make - E.G. from the manual
(https://www.gnu.org/software/make/manual/html_node/Environment.html):

Variables in make can come from the environment in which make is run.  Every
environment variable that make sees when it starts up is transformed into a
make variable with the same name and value

So we end up in make with TOOLCHAIN_VERSION set to the value of the
environment variable.  As virtual packages do not have a version, there is
no explicit TOOLCHAIN_VERSION = ..  line in toolchain.mk overriding this
value, and the logic in package/pkg-generic.mk sets a default value for
TOOLCHAIN_SOURCE when TOOLCHAIN_VERSION is set, and finally errors out as
TOOLCHAIN_SITE isn't set.

As a workaround, explicitly set <pkg>_VERSION and <pkg>_SOURCE to the empty
string in the virtual package infrastructure.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>