package/pkg-meson.mk: allow packages to add extra [binaries]

package/pkg-meson.mk: allow packages to add extra [binaries]

meson does not allow passing path to helper programs (e.g. pkgconfig)
using variables in the environment. Instead, it insists that those paths
be defined in the cross-compilation.conf file, in the [binaries]
section [0]

As such, allow packages to declare such a list of arbitrary entries to
add in the [binaries] section.

[0] https://github.com/mesonbuild/meson/issues/3327 for the LLVM_CONFIG
    example, which we'll address in a follow-up patch.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>