package/perl: fix configure when BR2_VERSION_FULL contains a '/'

package/perl: fix configure when BR2_VERSION_FULL contains a '/'

When BR2_VERSION_FULL contains one or more '/', injection our version
in the perl patch-level fails:

    /usr/bin/sed: -e expression #1, char 27: unknown option to `s'

When the build is done in a git tree, and HEAD is a tag, BR2_VERSION_FULL
will contain that tag name. Even if not widely common, it is not unusual
for a tag to contain a '/', and this is perfectly legit in git.

So, mangle BR2_VERSION_FULL to escape all '/' with a backslash '\', so
that the sed expression is correct, and so that we eventually have a
correct pathclevel string in perl's --version output.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>