change kernel build scripts to use defconfig

Here are the changes to the build scripts that are needed to use the changes in mindboards/ev3dev-kernel#13.

I did not mess with the rootfs/sd card scripts, so I was probably overzealous on the .gitignore cleanup.

The link for README.md was not working with github, so I make one just for this repo.

```build-kernel```: Change _ to - for consistency. First, it checks for an existing .config file. If one is not found, it uses the ev3dev_defconfig file to create a new one. With no args, it builds the kernel and copies the uImage and modules to a local directory. With args, the args are passed to make so you have access to anything that the kernel makefile can do.

Since we have ev3dev_defconfig in the kernel source now, we don't need the ev3dev.config file. Having the .config in 2 different repos was problematic anyway. They would get out of sync because there is nothing (like submodules) to link the 2 repos.

```update-defconfig``` updates the ev3dev_defconfig file with the current .config.

I renamed build_kconfig to menuconfig because that is really all it is.

I also added back in a script for downloading and installing the required build tools.