NuttX: link program/application only once into combined system image.

NuttX: link program/application only once into combined system image.

When given program had been built as builtin command, it was linked
as builtin relocatable object (PBI) which is linked together
with NuttX system libraries and program object files (again) into
target system image. But this way the actual program objects
are linked twice into OS image.

This change ensures that actual program PBI is not linked in
and alias <progname>_main is created to reference main()
from builtin programs table.

Signed-off-by: Pavel Pisa <ppisa@pikron.com>