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>
Drop U-Boot specifig CONFIG_ switch

The functions for managing the U-Boot's environment
are taken from U-Boot project and use several switches
that are defined when U-Boot is configured for a specific board.

swupdate does not use them, and there is not
such board configuration. The generated default environment
is then wrong and should not be used at all.
swupdate always need that the environment is described
by fw_env.config and other cases are dropped from code.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CONFIG_UBOOT cannot be disabled

Disabling CONFIG_UBOOT is not compile clean.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Fix return value in shell scripts

Signed-off-by: Stefano Babic <sbabic@denx.de>
doc: change first page on website

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