ColdFire: Update the lds file for M54418TWR board.

The M54418TWR lds file  need to update since commit:

8b493a52

common: Discard the __u_boot_cmd section

The command declaration now uses the new LG-array method to generate
list of commands. Thus the __u_boot_cmd section is now superseded and
redundant and therefore can be removed. Also, remove externed symbols
associated with this section from include/command.h .

Signed-off-by: Jason Jin <Jason.jin@freescale.com>