[mkbundle] Add support for generating monodroid compatible mkbundle code at runtime...

[mkbundle] Add support for generating monodroid compatible mkbundle code at runtime (#4728)

monodroid used to build its own version of mkbundle since it
requires slightly different code than the desktop. This
logic was protected in a #if block. Now that monodroid is
trying to make use of the shipped mkbundle rather than use
its own we need to turn this logic on at runtime.

A new parameter has been added

	--bundled-header

When this option is passed, the code which skips including

	mono/metadata/mono-config.h

will be skipped in favour of defining the required structures
and entry points directly.