package/infra: fix downloading versions with a slash

Since cset 54456cc6 (infra: consistently use double dollar signs...), it
is no longer possible to download versions with a slash in it, because
the _DL_VERSION variable is second-expanded, but the _VERSION variable
is immediately-expanded to have '/' substitued with '_'.

So, _DL_VERSION is only expanded at the time it is evaluated, and by
that time, we've lost the slashes in _VERSION.

Make the _DL_VERSION variables immediately-evaluated.

Fixes:
    https://bugs.busybox.net/show_bug.cgi?id=7328



Reported-by: Vincent Stehlé <vincent.stehle@freescale.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc:  Vincent Stehlé <vincent.stehle@freescale.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>