Fix inclusion of new object files

Fix inclusion of new object files

Right now, there's a mixup over the xen/ part of the path for new files.

  + NEW_FILES=./arch/x86/lp-hooks.o
  + for i in '$NEW_FILES'
  ++ dirname ./arch/x86/lp-hooks.o
  + mkdir -p output/./arch/x86
  + cp patched/./arch/x86/lp-hooks.o output/./arch/x86/lp-hooks.o
  cp: cannot stat 'patched/./arch/x86/lp-hooks.o': No such file or directory

Alter the `cd` and `find` runes to use paths relative to the root of the
different source trees.

This now results in:

  + NEW_FILES=xen/arch/x86/lp-hooks.o
  + for i in '$NEW_FILES'
  ++ dirname xen/arch/x86/lp-hooks.o
  + mkdir -p output/xen/arch/x86
  + cp patched/xen/arch/x86/lp-hooks.o output/xen/arch/x86/lp-hooks.o

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>