Allow to use a different interpreter for scripts

Scripts are written in LUA. The patch makes possible
to use a different handler as LUA, for example
using shell scripts.

However, LUA remains the best way in swupdate for
pre- and postinstall scripts.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Fix max filename in strem_interface.c

Signed-off-by: Stefano Babic <sbabic@denx.de>
Let swupdate to handle SIGCHLD properly

The Webserver sets to ignore the SIGCHLD signal,
but then any call to system() reports "no child process".
Let the process to handle the signals properly.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Added handler for shell scripts

The handler lets to have a script as shell script
instead of LUA. The type in the sw-description
must be defined as "shellscript".

Signed-off-by: Stefano Babic <sbabic@denx.de>