kbuild: change *FLAGS_<basetarget>.o to take the path relative to $(obj)

Kbuild provides per-file compiler flag addition/removal:

  CFLAGS_<basetarget>.o
  CFLAGS_REMOVE_<basetarget>.o
  AFLAGS_<basetarget>.o
  AFLAGS_REMOVE_<basetarget>.o
  CPPFLAGS_<basetarget>.lds
  HOSTCFLAGS_<basetarget>.o
  HOSTCXXFLAGS_<basetarget>.o

The <basetarget> is the filename of the target with its directory and
suffix stripped.

This syntax comes into a trouble when two files with the same basename
appear in one Makefile, for example:

  obj-y += foo.o
  obj-y += dir/foo.o
  CFLAGS_foo.o := <some-flags>

Here, the <some-flags> applies to both foo.o and dir/foo.o

The real world problem is:

  scripts/kconfig/util.c
  scripts/kconfig/lxdialog/util.c

Both files are compiled into scripts/kconfig/mconf, but only the
latter should be given with the ncurses flags.

It is more sensible to use the relative path to the Makefile, like this:

  obj-y += foo.o
  CFLAGS_foo.o := <some-flags>
  obj-y += dir/foo.o
  CFLAGS_dir/foo.o :=...