Commit 1272592e "powerpc: Use getenv_ulong() in place of getenv(),
strtoul" instroduced a build warning for some PPC systems:

board.c: In function 'board_init_r':
board.c:626: warning: unused variable 's'

Fix it.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>