Kbuild brought about many advantages for us but a significant
performance regression was reported by Simon Glass.

After some discussions and analysis, it turned out
its main cause is in $(call cc-option,...).

Historically, U-Boot parses all config.mk
(arch/*/config.mk and board/*/config.mk)
every time descending into subdirectories.
That means cc-options are evaluated over and over again.

$(call cc-option,...) is useful but costly.
So we want to evaluate them only in ./Makefile
and spl/Makefile and export compiler flags.

This commit changes the build system as follows:

  - Modify scripts/Makefile.build to not include config.mk
    Instead, add $(PLATFORM_CPPFLAGS) to asflags-y, ccflags-y,
    cppflags-y.

  - Export many variables
    Going forward, Kbuild will not parse config.mk files
    when it descends into subdirectories.
    If we want to set variables in config.mk and use them
    in subdirectories, they must be exported.

    This is the list of variables to get exported:
      PLATFORM_CPPFLAGS
      CPUDIR
      BOARDDIR
      OBJCOPYFLAGS
      LDFLAGS
      LDFLAGS_FINAL
        (used in nand_spl/board/*/*/Makefile)
      CONFIG_STANDALONE_LOAD_ADDR
        (used in examples/standalone/Makefile)
      SYM_PREFIX
        (used in examples/standalone/Makefile)
      RELFLAGS
        (used in examples/standalone/Makefile)

  - Delete CPPFLAGS
    This variable has been replaced with PLATFORM_CPPFLAGS

  - Copy gcclibdir from example/standalone/Makefile
    to arch/sparc/config.mk
    The reference in CONFIG_STANDALONE_LOAD_ADDR must be
    resolved before it is exported.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reported-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org> [on Sandbox]
Tested-by: Stephen Warren <swarren@nvidia.com> [on Tegra]

Unlike Linux Kernel, U-Boot historically had *.dts files under
board/$(VENDOR)/dts/ and *.dtsi files under arch/$(ARCH)/dts/.

I think arch/$(ARCH)/dts dicretory is a better location
to store both *.dts and *.dtsi files.

For example, before this commit, board/xilinx/dts directory
had both Microblaze dts (microblaze-generic.dts) and
ARM dts (zynq-*.dts), which are totally unrelated.

This commit moves *.dts to arch/$(ARCH)/dts/ directories,
allowing us to describe nicely mutiple DTBs generation in the next commit.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Useful rules in scripts/Makefile.lib allows us to easily
generate a device tree blob and wrap it in assembly code.

We do not need to parse a linker script to get output format and arch.

This commit deletes ./u-boot.dtb since it is a copy of dts/dt.dtb.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Now we are ready to switch over to real Kbuild.

This commit disables temporary scripts:
  scripts/{Makefile.build.tmp, Makefile.host.tmp}
and enables real Kbuild scripts:
  scripts/{Makefile.build,Makefile.host,Makefile.lib}.

This switch is triggered by the line in scripts/Kbuild.include
  -build := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.build.tmp obj
  +build := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.build obj

We need to adjust some build scripts for U-Boot.
But smaller amount of modification is preferable.

Additionally, we need to fix compiler flags which are
locally added or removed.

In Kbuild, it is not allowed to change CFLAGS locally.
Instead, ccflags-y, asflags-y, cppflags-y,
CFLAGS_$(basetarget).o, CFLAGS_REMOVE_$(basetarget).o
are prepared for that purpose.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Gerhard Sittig <gsi@denx.de>

This commit imports build scripts from Linux Kernel v3.13
as they are.

I know they include some trailing spaces
but I am intentionally keeping them untouched.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>