package/pkg-meson: allow meson packages to specify custom build rules

package/pkg-meson: allow meson packages to specify custom build rules

Some packages may want to build only specific targets, instead of the
default.

So, allow them to provide FOO_NINJA_OPTS (not really options, but we
just mimicked the naming we already have for autotools packages).

Update the manual accordingly.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>