layerscape: sysupgrade: get rid of platform_pre_upgrade()

1) RAMFS_COPY_BIN and RAMFS_COPY_DATA can be defined at top of the file
   like it's done for all other targets.
2) fw_printenv.lock can be created one step later in the
   platform_do_upgrade(). It seems to be working well on many other
   targets.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>