hikey+debian: Add atf-fastboot target

atf-fastboot is a component of the new l-loader.bin at
https://github.com/96boards-hikey/l-loader


branch: testing/hikey960_v1.2
that provides fastboot functionality to bl1.bin

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