pkg-generic: Fix host _DL_VERSION corner case

In the case when a package has a host version, the package is dependent
on the host version, and the version contains a '/', the host version
does not evaluate properly. The host version will contain a '_' instead
of a '/', resulting in a failed download. To solve this corner case, add
a check to see if the _DL_VERSION of the package has been defined before
defining the host _DL_VERSION. If the package _DL_VERSION has not been
defined yet, then the version string has not been formatted yet and is
still good to use.

[Arnout: further simplify things by lifting the override over _VERSION
out of the condition - it is always the same.]

This error occured on a package in a BR2_EXTERNAL that uses a git repo
for its remote storage with '/' in the tag names. I do not believe this
affects any packages in the Buildroot mainline but it could in the
future.

[Arnout: rebase on master, fix existing whitespace error in the else
branch.]

Signed-off-by: Clayton Shotwell <clayton.shotwell@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>