uboot-imx

config: remove redundant CONFIG_SYS_PROMPT defines

Since commit 0defddc851ed (config: Add a default CONFIG_SYS_PROMPT),
each board header does not need to define CONFIG_SYS_PROMPT
as long as it uses the default prompt "=> ".

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>