Fix pxe network boot

*This commit fixes pxe network boot and enables nixos tarball to be bootable from network.*

**Changes:**
- Override correct kernel, tarball is using 3.4, but old overriden kernel was 3.2, so it was unaffective
- Add additional built-in kernel config options so that kernel compiles and pxe boot actually works
- Make mounted nfs image read-write
- Add R8169 realtek NIC driver, which a lot of motherboards use(i have 3 totally different laptops and all of them use this driver)

PS: If you wonder what CONFIG_UNIX kernel options does and why it's needed here http://www.linux-mips.org/archives/linux-mips/2006-11/msg00113.html