Choose config with TARGET variable
Usage when no command specified
Add update-image/-script internal commands
Add update internal command
Add $RPI_XC_ARGS which gets passed to docker

Closes #6
Rename TARGET to RPI_XC_TARGET

Make all external vars consistent
Source config file if it exists

Specified in RPI_XC_CONFIG or ~/.rpi-xc

Closes #5
Add configuration section to README.md
Document RPI_XC_TARGET in README.md
Document build-in commands in README.md