nios2: Preconfigure $loadaddr variable

Preset the $loadaddr environment variable to some sane default, let's
say half of the RAM. This variable is where the kernel is loaded using
all sorts of .*load commands, so it's convenient to have it set.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Thomas Chou <thomas@wytron.com.tw>