duktape for Unikraft

This is a port of duktape to Unikraft. You will need newlib
for it to work, so please ensure you add the following line to the
LIBS variable in the Makefile:

…$(UK_LIBS)/newlib:$(UK_LIBS)/duktape…

For now we only support interactive mode. To use it, please make sure
to call duktape_main from your application. Also, please make sure to
allocate enough memory (e.g., 8-16 MBs) for duktpace to run).

Please refer to the README.md as well as the documentation in the doc/
subdirectory of the main unikraft repository for further information.