Added arch_endian.h for ARM
Disable memory test

Everything seems to be working now and the output clutters up the
console logs.
Added ARM grant table initialisation

On x86, we get told which pages to use, but on ARM we get the base
address in the FDT and have to map each page.
Networking on ARM

Set MEM_ALIGNMENT=4 to request word-aligned memory access. However, this
does not cover all cases in lwIP, so also configure the CPU to allow
unaligned accesses.