Add Support for FreeRTOS for OpenAMP RPU Apps

[bentheredonethat]

Today OpenAMP RPU apps have hard-coded helper.c and example main functions that assume they are for baremetal OS.

Below is proposal for design to add Xilinx-AMD supported FreeRTOS additions:

1. Move GIC Init logic that today is always assumed for baremetal to its own file called gic_init.c and pick up either baremetal or freertos gic_init.c. Link against either gic_init.c::app_gic_init() based on the target OS in CMake. See below diagram for changes.

2. OS-specific main routine - For each of the demos add OS specific files with the main routine to link against. Link against apps/example///main.c::main() routine based on target OS. See below diagram:

[bentheredonethat]

Apologies for formatting of the diagrams...

• white rectangle is existing dir
• white round edged rectangle is existing source file
• yellow rectangle is new dir
• yellow round edged rectangle is new source file

CC @arnopo @tnmysh @wmamills

[arnopo]

Hi @bentheredonethat
Before discussing such evolution the priority would be to move apps in https://github.com/OpenAMP/openamp-system-reference
do you discuss the topic internally with @tnmysh ?