ppc4xx: Fix default environment for AMCC boards

Commit 0ad7f095 [ppc4xx: cleanup
default environment for AMCC boards] broke the default env for
many PPC4xx boards. The '\0' character got removed at the end
of some environment commands like "update". This patch adds the
missing '\0' characters again.

Signed-off-by: Stefan Roese <sr@denx.de>