Now allowing definition of target via the ARSCONS_TARGET environment variable

Sometimes it happens to have more than one sketch in a folder, or to
have the need to name the sketch differently from the containing folder.
So, I added support for specifying its name by setting the ARSCONS_TARGET
environment variable / scons argument.
Added extra ld search path to fix couple of problems in gentoo amd64

* Workaround for /usr/libexec/gcc/avr/ld: cannot open linker script file ldscripts/avr5.x: No such file or directory
* Workaround for /usr/libexec/gcc/avr/ld: crtm168.o: No such file: No such file or directory