RN: Add previous_release_name as global variable

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
RN: Extend the introduction section

Add
 -What is FSL community BSP
 -What project is not
 -Motivation
 -What to expect
 -Braching schema
 -Upstreaming cycle
 -Differences between this project and Freesca official release

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
RN: Add the product family description section

This section should describe the different product
families covered.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>