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>