nios2: Make STANDALONE_LOAD_ADDR configurable per board

Follow commit 8ae86b76
which changed the variable name.

Fix this error,
nios2-elf-ld: invalid hex number `-o'

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>