This function extracts build target from command line.

Bug: 900161
Change-Id: I38d37689a42437de76933ea3c9afb9def5a1253c
Reviewed-on: https://chromium-review.googlesource.com/c/1381911
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Shinya Kawanaka <shinyak@chromium.org>