packages/*.mk: Remove duplicate options

These flags are already defined in pkg-autotools.mk.
There is no need to have them twice.

[Thomas: restore += instead of = in the opus package, as suggested by
Yann E. Morin.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>