Every makefile in sub directories has common lines
at the top and the bottom.
This commit pushes the common parts into script/Makefile.build.

Going forward sub-makefiles only need to describe this part:

    COBJS := ...
    COBJS += ...
    SOBJS := ...

But using obj-y is preferable to prepare for switching to Kbuild.

The conventional (non-Kbuild) Makefile style is still supported.
This is achieved by greping the Makefile before entering into it.
U-Boot conventional sub makefiles always include some other makefiles.
So the build system searches a line beginning with "include" keyword
in the makefile in order to distinguish which style it is.
If the Makefile include a "include" line, we assume it is a conventional
U-Boot style. Otherwise, it is treated as a Kbuild-style makefile.

With this tweak, we can switch sub-makefiles
from U-Boot style to Kbuild style little by little.

obj-y := foo/
syntax (descending into the sub directory) is not supportd yet.
It will be implemented in the upcomming commit.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>