Pre-allocate secure-side context structures

Pre-allocate secure-side context structures

This commit improves ARMv8-M security by pre-allocating secure-side task
context structures and changing how tasks reference a secure-side
context structure when calling a secure function. The new configuration
constant secureconfigMAX_SECURE_CONTEXTS sets the number of secure
context structures to pre-allocate. secureconfigMAX_SECURE_CONTEXTS
defaults to 8 if left undefined.

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