Update bootutil to reduce amount of global state

The boot loader state currently exists as global variable (boot_data)
which is accessed by all routines; this updates all routines that
require access to the state to receive it by parameter.

Variables that are declared as "static" (globals) were #ifdef'ed to
only use static when building a native bootloader, to avoid stack
allocation. When bootutil is built to run in the simulator they are
stack allocated to avoid thread share.

Signed-off-by: Fabio Utzig <utzig@apache.org>