Add descriptions about U-Boot SPL feature and how to build and run.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Cc: Alan Kao <alankao@andestech.com>

These board index.rst files are missing in previous html doc patch
series, hence it causes some build warnings. Add those files.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

This converts README.AX25 and README.ae350 plain text documentation
to reST format, merges them into one ax25-ae350 doc, and adds it to
Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>