Based on the following Linux commits:

 - 54a702f70589 ("kbuild: mark $(targets) as .SECONDARY and remove
   .PRECIOUS markers")

 - 8e9b61b293d9 ("kbuild: move .SECONDARY special target to
   Kbuild.include")

This consolidates those two commits and adjusts commit log for barebox.

GNU Make automatically deletes intermediate files that are updated
in a chain of pattern rules.

Example 1) %.dtb.o <- %.dtb.S <- %.dtb <- %.dts
Example 2) %.o <- %.c <- %.c_shipped

arch/arm/crypto/Makefile marks such targets as .PRECIOUS to prevent Make
from deleting them, but the correct way is to use .SECONDARY.

  .SECONDARY
    Prerequisites of this special target are treated as intermediate
    files but are never automatically deleted.

  .PRECIOUS
    When make is interrupted during execution, it may delete the target
    file it is updating if the file was modified since make started.
    If you mark the file as precious, make will never delete the file
    if interrupted.

Both can avoid deletion of intermediate files, but the difference is
the behavior when Make is interrupted; .SECONDARY deletes the target,
but .PRECIOUS does not.

The use of .PRECIOUS is relatively rare since we do not want to keep
partially constructed (possibly corrupted) targets.

.SECONDARY with no prerequisites causes all targets to be treated as
secondary. This agrees the policy of Kbuild.

scripts/Kbuild.include seems a suitable place to add it because it is
included from almost all sub-makes.

I deleted 'SECONDARY' (no dot) from images/Makefile, which I guess
a typo.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

linux arm sha256 current
$ ls -al build/versatilpb/arch/arm/pbl/zbarebox.bin
 -rw-r--r-- 1 root root 207786 Mar 24 13:23 build/versatilpb/arch/arm/pbl/zbarebox.bin

linux arm v4 asm implementation for sha256
$ ls -al build/versatilpb/arch/arm/pbl/zbarebox.bin
 -rw-r--r-- 1 root root 205007 Mar 24 16:47 build/versatilpb/arch/arm/pbl/zbarebox.bin

we win 2779 bytes and speed cf code

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

from Linux 3.9

linux generic implementation

$ ls -al build/versatilpb/arch/arm/pbl/zbarebox.bin
-rw-r--r-- 1 root root 210829 Mar 24 13:21 build/versatilpb/arch/arm/pbl/zbarebox.bin

linux arm v4 asm implementation
$ ls -al build/versatilpb/arch/arm/pbl/zbarebox.bin
-rw-r--r-- 1 root root 207786 Mar 24 13:23 build/versatilpb/arch/arm/pbl/zbarebox.bin

we win 3043 bytes and speed cf code

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>