U-Boot has imported various source files from other projects,
mostly Linux.

Something like

  #ifdef __UBOOT__
    [ modification for U-Boot ]
  #else
    [ original code ]
  #endif

is an often used strategy for clarification of adjusted parts,
that is, easier re-sync in future.

Instead of defining __UBOOT__ in each source file,
passing it from the top Makefile would be easier.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>

from linux 3.14:

commit 455c6fdbd219161bd09b1165f11699d6d73de11c
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun Mar 30 20:40:15 2014 -0700

    Linux 3.14

Needed for the MTD/UBI/UBIFS resync

Just copied the files from Linux, and added in the c-file
the "#define __UBOOT__" for adding U-Boot special code. In
this case we use this just for adding including U-Boot
headers.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Sergey Lapin <slapin@ossfans.org>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Tom Rini <trini@ti.com>