kbuild: add a makefile macro useful with per-image config options

Commit e02ee2548afe ("kconfig: switch to single .config
configuration") made the configuration itself pretty simple,
instead, we lost the way to systematically enable/disable config
options for each image independently.

Our current strategy is, put entries into Makefile.spl for options
we need separate enabling, or once enable the options globally in
Kconfig and then undef them in Makefile.uncmd_spl if we do not want
to compile the features for SPL at all.  Things are getting really
messy.  Besides, "ifdef CONFIG_SPL_BUILD" are sprinkled everywhere
in makefiles.

This commit adds a variable to help describe makefile simpler.

$(SPL_) evaluates to "SPL_" during the SPL build, while to an empty
string during building U-boot proper.

So, you can write

  obj-$(CONFIG_$(SPL_)FOO) += foo.o

instead of

  ifdef CONFIG_SPL_BUILD
  obj-$(CONFIG_SPL_FOO) += foo.o
  else
  obj-$(CONFIG_FOO) += foo.o
  endif

If CONFIG_SPL_FOO does not exist in Kconfig, it is equivalent to

  ifndef CONFIG_SPL_BUILD
  obj-$(CONFIG_SPL_FOO) += foo.o
  endif

This is the pattern we often see in our current makefiles.

To take advantage of this macro, we should prefix SPL_ for the SPL
version of the option when we need independent control between
U-boot and SPL.  With this naming scheme, I hope our makefiles will
be much simplified.

It means we want to rename existing config options as follows
in the long run:

  CONFIG_SPL_SERIAL_SUPPORT     -> CONFIG_SPL_SERIAL
  CONFIG_SPL_I2C_SUPPORT        -> CONFIG_SPL_I2C
  CONFIG_SPL_GPIO_SUPPORT       -> CONFIG_SPL_GPIO
  CONFIG_SPL_SPI_SUPPORT        -> CONFIG_SPL_SPI
  CONFIG_SPL_DISABLE_OF_CONTROL -> CONFIG_SPL_OF_CONTROL
                                      (inverting the logic)

Then drivers/Makefile would be re-worked as follows:

  obj-$(CONFIG_$(SPL_)SERIAL)  += serial/
  obj-$(CONFIG_$(SPL_)I2C)     += i2c/
  obj-$(CONFIG_$(SPL_)GPIO)    += gpio/
  obj-$(CONFIG_$(SPL_)SPI)     += spi/
     ...

Eventually, SPL-specialized entries in Makefile.spl would go away.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>