buildroot

zynq: unify readme.txt for all Zynq boards

The readme.txt for the three boards are almost the same, so merge
them into board/zynq/readme.txt.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>