Automatically pull in library dependencies.

Now if you have USBHost or net/eth in your MBED_LIBS, the build system
will automatically link in the other libraries required (ie. rtos).

Also converted makefile.creole to only use Unix line endings in the
process of making these updates.
Refactor makefiles to reduce redundancy

Pulled repeated code such as that for setting the compiler flags out of
many makefiles and placed once in device-libs.mk

Now just assemble all assembly language source files via GCC so that
they can be pre-processed if need be.  Now just support the AS_FLAGS
variable in the user's makefile and dropped AS_GCFLAGS.

A lot of the code in the makefiles which build libraries (mbed.a,
rtos.a, etc) had a lot of code in common.  I have pulled this out into
lib-common.mk and include from other library creation makefiles.
Renamed makefiles.

Just simplified the names of the included makefiles.  Reduced the
usage of the word 'device' in the filenames.