tools: mkeficapsule: use pkg-config for each lib separat

Call pkg-config for each library individually.
This improves fallback handling.

Suggested-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Fixes: 31a7688c ("tools: mkeficapsule: use pkg-config to get -luuid and -lgnutls")
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>