Slight change in EDK2 compiling

Slightly changes the way EDK2 is compiled by sourcing edksetup.sh
instead of just running it. When compiling EDK2 that script is supposed
to be sourced.

Also records a dependency to iasl in recend EDK2 builds

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>