Rename include/md5.h to include/u-boot/md5.h

Some systems have md5.h installed in /usr/include/. This isn't the
desired file (we want the one in include/md5.h). This will avoid the
conflict. This fixes the host tools building problem by creating a new
directory for U-Boot specific header files.

[Patch by Andy Fleming, modified to use separate directory by Wolfgang
Denk]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Acked-by: Timur Tabi <timur@freescale.com>