arm: mx5: Fix NAND image generation

arm: mx5: Fix NAND image generation

The echo -ne "\xNN" does not work in certain bourne-compatible shells, like
dash. The recommended way of hex->char conversion is using printf(1), but
there is a pitfall here. The GNU printf does support "\xNN" format, but
according to the opengroup documentation, this is not part of POSIX. The
POSIX printf only defines "\NNN" where N is octal. Thus, for the sake of
compatibility, we use that.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>