km: replace hardcoded address for imported environment

km: replace hardcoded address for imported environment

Instead of using an hard coded address, make use of an
already defined address for importing the environment
for ramfs and nfs boot. This allows boards having different
mapping to use the same code.

CC: Heiko Schocher <hs@denx.de>
CC: Tom Rini <trini@konsulko.com>
Signed-off-by: Matteo Ghidoni <matteo.ghidoni@hitachi-powergrids.com>
Signed-off-by: Holger Brunck <holger.brunck@hitachi-powergrids.com>