mkpasswd: change version from 'undefined' to 'buildroot-$(BR2_VERSION)'

The sources of the mkpasswd package are shipped with Buildroot, rather than
downloaded from an external location. As a result, no explicit version is
defined, causing build messages and build directory to show 'undefined' as
version.

This patch sets the version for mkpasswd to 'buildroot-$(BR2_VERSION), which
would for example expand to 'buildroot-2014.05'.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>