mvebu: fix sysupgrade

mvebu: fix sysupgrade

mvebu was modifying RAMFS_COPY_BIN and RAMFS_COPY_DATA from a
sysupgrade_pre_upgrade hook. As the ramfs is created from stage2, this
did not have an effect anymore after the staged sysupgrade changes.

As it doesn't really hurt to copy fw_printenv and fw_setenv
unconditionally, simply add them in /lib/upgrade/platform.sh, so stage2
will see them.

Config copying is moved to a function called by platform_copy_config, where
it belongs.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Fixes: FS#821
Fixes: 30f61a34b4cf "base-files: always use staged sysupgrade"