samples: Add a sample for runtime chosen image

A sample for runtime chose image on FRDM K64F. It provides implementation
for boot_go and flash area id related hooks, showing how an external
module implementing hooks can influence which images end up being booted
on the platform.

In this sample, one can influence from which slot image will be loaded
by pressing a button on the board.

For more details on how to build and test the samples, check the
provided README.md.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>
Signed-off-by: Tom Burdick <thomas.burdick@intel.com>