Add filename_from_src property to prebuilt_etc

Base name of the output file of a prebuilt_etc is by default the module
name. This default behavior can be customized by either via the
'filename' property or the new 'filename_from_src' property. The former
explicitly sets the base name while the latter makes the file name to be
that of the source file.

Test: m (prebuilt_etc_test added)
Change-Id: Ic2900417bda62993f6de2612d993234b82b74479