app/test: support resources externally linked

app/test: support resources externally linked

To include resources from other source that the C source code we
can take advantage of the objcopy behaviour, i.e. packing of an
arbitrary file as an object file that is linked to the target program.

A linked object file is always accessible as a pair

extern const char beg_<name>;
extern const char end_<name>;
(extern const char siz_<name>;)

A unit test that packs the resource.c source file is included.

Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>