[U-Boot] [RESEND PATCH v2 4/4] kbuild: improve Kbuild speed

[Masahiro Yamada]

Hello Tom,


> 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:

Please hold applying this patch.
I have found a bug in this patch.

microblaze-generic board will be broken with this patch.
( microblaze-linux-gcc  fails in building this board,
 so I missed this bug when I tested this series.)

I will post version 3 tomorrow.

Best Regards
Masahiro Yamada