Prevent unprivileged task from altering MPU configuration (#227)

Prevent unprivileged task from altering MPU configuration (#227)

This change removes the FreeRTOS System Calls (aka MPU wrappers) for the
following kernel APIs:
- xTaskCreateRestricted
- xTaskCreateRestrictedStatic
- vTaskAllocateMPURegions

A system call allows an unprivileged task to execute a kernel API which
is otherwise accessible to privileged software only. The above 3 APIs
can create a new task with a different MPU configuration or alter the
MPU configuration of an existing task. This an be (mis)used by an
unprivileged task to grant itself access to a region which it does not
have access to.

Removing the system calls for these APIs ensures that an unprivileged
task cannot execute this APIs. If an unprivileged task attempts to
execute any of these API, it will result in a Memory Fault.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>