amcc-common.h: Use filenames from environment variables for update procedure.

Using a separate "u-boot" environment variable allows to easily
specify different filenames for the update procedure.  This is also in
line with many other board configurations defining an "update" script.

Signed-off-by: Detlev Zundel <dzu@denx.de>
Acked-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>