write_bootloader_message(<options>) should not reset stage field.

Commit bd56f159 switches to calling
write_bootloader_message(<options>) in get_args(), which
unintentionally resets the stage field thus breaks two-step OTAs.

This CL changes write_bootloader_message(<options>) to only set the
command field (to "boot-recovery") and the recovery field (with the
specified options).

Test: Apply a two-step package.
Change-Id: I6905918812c7d3402cc1524688079066a4d22d29