x86: use relative symlink for bzImage

Use relative symlinks so we can refer bzImage from different tree
structures aka via NFS.

Moved the creation of directory + symlink after a successful build of
the boot parts.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>