boot: mynewt: add support for the mcumgr echo command in serial boot mode

This allows user to enable echo for mcumgr command in serial boot.
Code was enabled in zephyr only so far.
Now mynewt build can also have this feature if enabled.

No code changes just option in mynewt build to enable existing
mcuboot feature.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>