spl: spl_nor: surround Linux-load code with #ifdef CONFIG_SPL_OS_BOOT

If CONFIG_SPL_NOR_SUPPORT is defined, spl_nor_load_image() requires
spl_start_uboot(), CONFIG_SYS_OS_BASE, CONFIG_SYS_SPL_ARGS_ADDR,
CONFIG_SYS_FDT_BASE to be defined even if users just want to run
U-Boot, not Linux.  This is inconvenient.

This patch is following the codying style of common/spl/spl_nand.c.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>