pkg-generic: add checks on deprecated variables FOO_BAR_OPT

This patch introduces a check for the usage of deprecated variable
FOO_BAR_OPT, and suggests to use FOO_BAR_OPTS instead.

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