Add default implementations of vApplicationGetIdleTaskMemory and vApplicationGetTimer...

Add default implementations of vApplicationGetIdleTaskMemory and vApplicationGetTimerTaskMemory (#790)

This PR introduces configKERNEL_PROVIDED_STATIC_MEMORY option
which the application can set to 1 to use the default implementations
of vApplicationGetIdleTaskMemory and vApplicationGetTimerTaskMemory
functions.

If the application enables static allocation (i.e. sets
configUSE_STATIC_ALLOCATION to 1) and does not provide the above 2
functions, it will result in linker error. The application has two options:

1. Set configKERNEL_PROVIDED_STATIC_MEMORY to 1 to use the default
    implementations of these functions.
2. Provide implementations of these 2 functions.

Note that default definitions are only available for non-MPU ports. The
reason is that the stack alignment requirements vary for different
architectures.