lx_emul: consolidate mm/page_alloc.c functions

[skalk]

@trimpim came up with the idea to concolidate the different driver specific implementations of page_frag_alloc* within lx_emul within issue genodelabs/genode-imx#12
@chelmuth stated it might be better to re-use mm/page_alloc.c from Linux.

[chelmuth]

My suggestion in genodelabs/genode-imx#12 was limited to the page_frag_*() functions.

[...] we shall go for porting the original implementation from mm/page_alloc.c. It's only 3 C functions.

Reusing mm/page_alloc.c is not an option in my opinion.