l4lx: allocate memory in chunks

When L4Linux tries to allocate a dataspace of the size of its physical
memory, this allocation can fail, because the 'l4re_ma_alloc()' function
in the 'l4lx' library always tries to allocate a contiguous dataspace of
the given size and there might be no contiguous free area left.

With this patch, memory gets allocated in chunks: if the size to be
allocated exceeds the configured chunk size, a managed dataspace gets
created and filled with multiple memory chunks of at most the chunk size.

The chunk size is 16M by default and can be configured in an l4linux
config node:

<config args="...">
	<ram chunk_size="16M"/>
</config>

Fixes #695.