Here is the v2 of this series. The first 5 patches are just cleanup: some
exported headers were still under a non-uapi directory.
The patch 6 was spotted by code review: there is no in-tree user of this
functionality.
The last patch remove the use of header-y. Now all files under an uapi
directory are exported.

asm is a bit special, most of architectures export asm/<arch>/include/uapi/asm
only, but there is two exceptions:
 - cris which exports arch/cris/include/uapi/arch-v[10|32];
 - tile which exports arch/tile/include/uapi/arch.
Because I don't know if the output of 'make headers_install_all' can be changed,
I introduce subdir-y in Kbuild file. The headers_install_all target copies all
asm/<arch>/include/uapi/asm to usr/include/asm-<arch> but
arch/cris/include/uapi/arch-v[10|32] and arch/tile/include/uapi/arch are not
prefixed (they are put asis in usr/include/). If it's acceptable to modify the
output of 'make headers_install_all' to export asm headers in
usr/include/asm-<arch>/asm, then I could remove this new subdir-y and exports
everything under arch/<arch>/include/uapi/.

Note also that exported files for asm are a mix of files listed by:
 - include/uapi/asm-generic/Kbuild.asm;
 - arch/x86/include/uapi/asm/Kbuild;
 - arch/x86/include/asm/Kbuild.
This complicates a lot the processing (arch/x86/include/asm/Kbuild is also
used by scripts/Makefile.asm-generic).

This series has been tested with a 'make headers_install' on x86 and a
'make headers_install_all'. I've checked the result of both commands.

This patch is built against linus tree. I don't know if it should be
made against antoher tree.

Comments are welcomed,
Nicolas