Prepare for compiling FORB "components" for RTEMS

Prepare for compiling FORB "components" for RTEMS

On Linux, FORB modules are normal shared libraries. The RTEMS support
for shared libraries does not seem to be mature so we use static libraries
for the components. These will have to be linked manually to the final
application.

The FORB components should use FORB_MAIN macro to define their main
function (previously called forb_main).

In this commit I only convert dummy resource to this new syntax. The rest
is leaved for Petr :-)

It will be necessary to implement an equivalent of forbrun for RTEMS. My
idea is to introduce a special ELF section with pointers to FORB_MAIN
functions and these functions will be called at the end of forb_init().