add more "how to" doc for zephyr targets

Description

Looks like the description is too dry. It needs to be more guide.

I observe that:
it is not clear to user how-to compile proper image-application and how to create the image for upgarde
https://github.com/runtimeco/mcuboot/blob/master/README-zephyr.rst#building-applications-for-the-bootloader
Prefered here is examle of what should be added to the project cmake file and how a-board-overlay.dts should looks like. For instance we can redact this somehow:
```
Put into the project's top cmake the path to the overlay file:

set(DTC_OVERLAY_FILE "${CMAKE_CURRENT_SOURCE_DIR}/board-dts.overlay") before including boilerplate.cmake, File should looks for instance like this:
set(CONF_FILE "prj.conf")

set(DTC_OVERLAY_FILE "${CMAKE_CURRENT_SOURCE_DIR}/dts.overlay")

include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
project(NONE)

target_sources(app PRIVATE src/main.c)

add the file <a-board.overlay> with payload:

/ {
chosen {
zephyr,code-partition = &slot0_partition;
};
};
```

for: https://github.com/runtimeco/mcuboot/blob/master/README-zephyr.rst#signing-the-application need to exaplane --pad imagetool option user case (manually triggered test image swap).

Status

Assignee

Carles Cufi

Reporter

Andrzej Puzdrowski

Labels

None

Priority

Medium