bootstd: rauc: extend and fix doc to reflect real flow

bootstd: rauc: extend and fix doc to reflect real flow

The documentation of bootmeth rauc in some aspects does not reflect the
real program flow. Specifically the reset of boot tries in case of "no
more slots found" is incorrect (it won't change BOOT_ORDER).
Also the search sequence for boot scripts was mixed and incomplete.
Fix these points in the documentation.

Explain the initial setup of any missing BOOT_ORDER and BOOT_x_LEFT
environment variables, and inform about BOOT_x_LEFT decrementing.

Signed-off-by: Andreas Pretzsch <apr@cn-eng.de>
Reviewed-by: Martin Schwan <m.schwan@phytec.de>