Add option to LDFLAGS to export LUA functions

Add -E option to LDFLAGS to load dinamically modules
at runtime. The modules need LUA function that must
be exported.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Fix return values in parse_config

Before adding network capability, each recognized error
stopped the software, that exited with error code.
Now the webserver must wait for further requests.

Fix parse_config to be sure that an error code
is returned if the parser fails.

Signed-off-by: Stefano Babic <sbabic@denx.de>
Add configurable external parser filename

Allow to configure the name of the external parser in LUA.
Fix parsing with external parser (tested with XML parser
in /examples).

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