SPRTL: remove rt_main

rt_main is an unnecessary layer that can be removed. Instead partition
entry point function can be called directly.
This simplifies the code and enables passing a parameter to the thread
using register r0. Previously passing parameter using r0 was impossible
because rt_main used r0.

Change-Id: I5e0d5278867d26bc79b39a8594c3d6ef0eb1e56d
Signed-off-by: Bohdan Hunko <Bohdan.Hunko@infineon.com>
Signed-off-by: Chris Brand <chris.brand@cypress.com>