vimrc: source vimrc.local if it exists

Signed-off-by: Stefan Naewe <stefan.naewe@gmail.com>
gitignore vimrc.local

Signed-off-by: Stefan Naewe <stefan.naewe@gmail.com>
vimrc: set backspace

Signed-off-by: Stefan Naewe <stefan.naewe@atlas-elektronik.com>