update_chroot: switch EMERGE_FLAGS to an array

Just a style thing.  Shouldn't be functionally different.

BUG=chromium:725571
TEST=precq passes

Change-Id: Ia93904dee70fdb532cd93caf2090016dfa4d0899
Reviewed-on: https://chromium-review.googlesource.com/538358
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
1 file changed