MFC 366897: Use a template assembly file to generate the embedded MFS.

This uses the .incbin directive to pull in the MFS image contents.
Using assembly directly ensures that symbols can be defined with the
name and properties (such as .size) desired without having to rename
symbols, etc. via a second objcopy invocation. Since it is compiled
by the C compiler driver, it also avoids the need for all of the
EMBEDFS* make variables.