Build core objects in subdirectory, fixes issue #82

Tidies up the build-* directory with a 4-line fix. Tested with alternate cores and multiple libraries (and even my 1.5 branch):

```
core/
libcore.a
libs/
weather24ard_txnet.d
weather24ard_txnet.eep
weather24ard_txnet.elf
weather24ard_txnet.hex
weather24ard_txnet.hex.sizeok
weather24ard_txnet.o
```