adding saveenv-command for bananapi r2

bananapi r2 can be booted from sd-card and emmc
saving the environment have to choose the storage
from which the device has booted

also the offset is set to 1MB to make sure env is written
to block "user data area" between uboot and first partition

https://www.fw-web.de/dokuwiki/lib/exe/fetch.php?cache=&media=bpi-r2:boot-structure.png



Signed-off-by: Frank Wunderlich <frank-w@public-files.de>