Added recipe for native GDB (still under development)

[ericlove]

You can compile it into the image by adding "gdb" it to core-image-riscv.bb, but it's not yet usable in qemu because of some GDB-internal problem, or possibly because of riscv-linux's ptrace support. I'll work on debugging that, but this recipe is independent of it.

It is, however, still a hack: the recipe acquire's Palmer's fixed-up GDB (https://github.com/riscv/riscv-binutils-gdb/) as part of the larger binutils repo, all of which is downloaded and compiled before GDB itself is extracted and installed. This isn't ideal, but there is no separate GDB repo.

[billasurendra]

Can any one tell me how to add new recipe for native GDB application debugging purpose.