Problem found: I have installed gnu utils and /usr/bin/gnu in PATH. So my script used first dd in path (/usr/bin/gnu/dd), which support my syntax.

/bin/dd (busybox) has real no support for conv and oflag.

Has somebody idea how to rewrite that script to work with default busybox?

Anyway, now I sent final patch series for rx51 u-boot support to mailinglist...