arch/powerpc/lib/board.c: fix build warning

Commit 1272592 "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>

arch/powerpc/lib/board.c

@@ -625,7 +625,9 @@ void board_init_r (gd_t *id, ulong dest_addr)
 {
 	bd_t *bd;
 	ulong malloc_start;
-
+#if defined(CONFIG_SYS_FLASH_CHECKSUM) || defined(CONFIG_CMD_NET)
+	char *s;
+#endif
 #ifndef CONFIG_SYS_NO_FLASH
 	ulong flash_size;
 #endif