sandbox: don't refer to symbol _init

GCC provides a symbol _init in crti.o on x86_64 and aarch64 but not on
RISC-V. The following lines leads to a build error for sandbox_defconfig on
RISC-V due to the missing symbol:

    common/board_f.c:269:
    #elif defined(CONFIG_SANDBOX) || defined(CONFIG_EFI_APP)
            gd->mon_len = (ulong)&_end - (ulong)_init;

The sandbox code is not copied into the memory allocated using mmap().
Hence we can safely use gd->mon_len = 0 to avoid the reference to _init.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>