Move CONFIG_UPDATE_TFTP code after CONFIG_PREBOOT

The auto-update feature (CONFIG_UPDATE_TFTP) requires that the env
variable serverip be set for the TFTP access. If DHCP is to be used
to get the serverip env variable, this doesn't work as DHCP happens
after the auto-update attempt has run. A solution is to run DHCP in
PREBOOT, but even this is too late.

To solve this, we move update_tftp() below the PREBOOT stuff.

Signed-off-by: Wolfgang Denk <wd@denx.de>