Kbuild: add LDFLAGS_STANDALONE

Introduce a new Makefile variable for passing LDFLAGS to standalone
programs. Currently the variable CONFIG_STANDALONE_LOAD_ADDR is
misued on some archs to pass a specific linker script.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Rick Chen <rick@andestech.com>