Define default values of macros before first use (#298)

Define default values of macros before first use (#298)

configSTACK_ALLOCATION_FROM_SEPARATE_HEAP was added recently in
https://github.com/FreeRTOS/FreeRTOS-Kernel/pull/267. This macro was
used in portable.h before its default value was defined, resulting in a
warning when built with -Wundef. This changes moves the default value
definition for configSTACK_ALLOCATION_FROM_SEPARATE_HEAP to portable.h
to ensure that it is defined before first use.

portUSING_MPU_WRAPPERS check in mpu_wrappers.h was updated in
https://github.com/FreeRTOS/FreeRTOS-Kernel/pull/285. The new check
results in a warning when built with -Wundef because
portUSING_MPU_WRAPPERS is not defined yet. This changes adds the default
value definition for portUSING_MPU_WRAPPERS to portable.h to ensure that
it is defined before first use.

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