[U-Boot] Making U-Boot smaller

[Simon Glass]

Hi,

(moved from thread "U-Boot PXA support")

We have of-platdata, which produces C data from the DT, for linking
into U-Boot. It saves libfdt and DT space. But we still have the DM
overhead.

We have binman which can insert values into the binary after
link-time. This is barely used at present, only for accessing the
location of things in flash.

Another thing is that every little tweak and feature adds a few bytes
and there are dozens of them in each release. It would be interesting
to build a board from 10 years ago (like PXA) and see where the growth
is. My bet is that we could add Kconfig options to disable extra
features (and enhancements of features) and make quite a difference.

For DM, I think it would be interesting to revisit and compare against
the initial release, and see if some features could be made optional
in SPL.

Finally I feel we could implement a single-device API for where
CONFIG_SPL_DM is not set. We could use the debug UART for serial, a
single instance of tiny MMC for MMC, etc.

Regards,
Simon