mynewt: Change main to mynewt_main

mynewt system for some time now uses mynewt_main() as
starting point called from startup code.
This changes function name main to mynewt_main but
provides backup main function that will be linked if
pre 1.12 mynewt-core is used with mcuboot

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