zmake: Add ro/rw elf to output/
zmake: Add ro/rw elf to output/

The elf files are very handy for debugging and should be included
in the build's output/ directory to make it easier to find.

BRANCH=none
BUG=none
TEST=zmake configure -b zephyr/projects/volteer
     see that build/zephyr/projects/volteer/output contains both
     zephyr.ro.elf and zephyr.rw.elf

Change-Id: Id7c13c4e7f5cf2b5b0e8b08d2df161695e485740
Signed-off-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2686984
Commit-Queue: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec