Update "Changes from Version 1 Kernel" to include a "How-To Port Apps" section

[zephyrbot]

Reported by Mark Linkmeyer:

As a Zephyr user, I need the "Changes from Version 1 Kernel" documentation (https://www.zephyrproject.org/doc/doc/kernel/overview/changes.html) to explain how to find and use the legacy.h file to port Zephyr applications from the version 1 kernel to the version 2 kernel. It should also include relevant examples to clearly show how this is done. Note that for ~99% of the APIs it should be a simple search and replace exercise to replace v1 kernel functions with v2 kernel functions. For the remaining 1%, the document must explain how to handle these cases too.

(Imported from Jira ZEP-1571)

[zephyrbot]

by Anas Nashif:

In the porting guide, take a sample, for example "synchronization" from samples/synchronization/ and from samples/legacy/synchronization/ and go through the steps of porting this from old kernel to the new one.
To make it more visual include snippets of the code in the documentation referencing the source code in the tree.

[zephyrbot]

by Anas Nashif:

will do a first pass on this, just to get us going and to have something online.

[zephyrbot]

by Mark Linkmeyer:

Correcting the priority field

[zephyrbot]

by David Kinder:

Is this https://www.zephyrproject.org/doc/porting/application.html "Work in Progress" document complete enough?