lib: utils: Add a macro in libfdt_env.h for strncmp

This commit add a macro to replace strncmp with sbi_strncmp.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>