build: provide suitable error for "both" libraries option

build: provide suitable error for "both" libraries option

Rather than having the DPDK configuration error out when linking apps
and examples when "both" is select for "default_library" option, we can
detect that setting earlier in the build config and provide a suitable
error message to the user.

Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Andrew Boyer <aboyer@pensando.io>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>