hikey+debian: Update and improve edk build commands

Use OpenPlatformPkg as required by
https://github.com/96boards-hikey/edk2


branch: testing/hikey960_v2.5

Use parallel instead of single execution

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>