This PR adds the possibility of specifying a memory error callback in
the "ram" element of the "config" section. The new configuration
attribute is called "memory_error_callback" and its value is the name of
a function with the signature "void f(size_t)" that will be called
automatically in the following cases:
- when malloc, calloc, or realloc (with the 'size' argument greater than
0) return NULL. The callback function will be called with the 'size'
argument of the allocation function.
- when the allocator encounters an internal error (only when using the
'multiple' allocator). The callback function will be called with the
argument set to 0.
For now, this feature is only availabsle when using the multiple
allocator or the simple allocator, not the built-in libc allocator.
Usage example (in the board configuration file):
```
config = {
ram = { memory_error_callback = "memory_error" }
}
```
- LPC288x platform converted to the new build system
- changed generation for the 'extmem' attribute
- added combined attributes (validated against two or more possible types)
