docs/manual: document $(FOO_PKGDIR)

In the manual, add a note that packages can use $(FOO_PKGDIR) to get the
path to the directory containg the .mk and Config.in files, if they need
it to e.g. copy files from there, like startup scripts, systemd units or
any other resource.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>