Makefile: fix expansion problems

The curly brackets were not expanded properly, at least in some shells.
Write out the full expansion instead.
src/Makefile: don't use kernel install target

Certain Linux distributions (e.g. Ubuntu) have system-wide hooks
installed that runs their own software in the "install" target for the
Linux kernel. This breaks BakkaLinux installation under those systems,
and if you run make as root, it might even break your running system.
Makefile: ensure all files in initramfs owned by root

If BakkaLinux was compiled as an ordinary user, files in the initramfs
image would not be owned by root (as expected), but by the compiling
user. As this doesn't make sense, tell cpio to change the owner to root
on all files.
linux: bump to linux-3.9
busybox: bump to 1.20.1

This includes a fix when compiling with a recent glibc, where gcc would
exit, complaining that

loginutils/passwd.c:188:12: error: ‘RLIMIT_FSIZE’ undeclared (first use
in this function)