Make sure that netboot kernel is writable before copying mfsroot into it and fail if not

[ivaniacono]

Hi, I've noticed that when I try to build a netboot image, the dd command in build_netboot silently fails when copying the mfsroot into the kernel file. The issue seems to be caused by the .netboot copy of the kernel file not having write permission. I'm experiencing this while building a firmware image for an MT7628 based board using the ralink cfg. This patch works for me, producing a firmware image with the mfsroot embedded into the kernel. Maybe am I missing something else? Thanks