Kbuild: standalone: do not ignore platform-specific OBJCOPYFLAGS

Currently the OBJCOPYFLAGS are cleared when assigning "-O srec"
or "-O binary" for standalone programs. All flags set by arch-specific
Makefiles are lost. This is bad if an arch demands arch-specific
flags for the objcopy step.

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