external/all: modify all of Make stubs inside external

external/all: modify all of Make stubs inside external

Previous build system does not support same files inside different modules.
i.e.
Makefile:184: target `platform.o' given more than once in the same rule.
Makefile:184: target `utils.o' given more than once in the same rule.
Makefile:184: target `base64.o' given more than once in the same rule.
Makefile:184: target `sha1.o' given more than once in the same rule.
Makefile:184: target `md5.o' given more than once in the same rule.

It causes in-complete compilations.
And it does not compile them as a different module.
So, it is changed like apps build system.