sandbox: eliminate unused functions from binaries

The sandbox should closely mimic other architectures.

Place each function or data in a separate section and let the linker
eliminate unused ones. This will reduce the binary size.

In the linker script mark that u_boot_sandbox_getopt are to be kept.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>