samples/Makefile: add explicit rule for linux-stm32
samples/Makefile: add explicit rule for linux-stm32

Modern (!) linkers treat .so files like .a: they just make one pass, so
-ldfu must be at the end of the command line. We cannot use the implicit
rule for executables anymore.

ciminaghi/libdfu,ciminaghi/libdfu,ciminaghi/libdfu