main: add merchanism to control UARTs on/off and path

Added a mechanism to allow the user to disable or enable interfaces to
UARTs from the command line. The interface also allows users to override
the default path for the TTY used for the UART so that it may be
programmatically controlled. The interface was implemented hopefully
generically enought to allow future peripherals to be configured as
well (e.g. -p uart0=off -p uart1=/tmp/uart1). This change also changed
the option to specify the storage of the flash memory to -s from -p.
main: support disabling UART if user requested

If the user requested the UART not be enabled then don't enable that
UART and don't crash when shutting down.
main: allow user supplied path for UART

Allow the user to supply a path for the UART and use that path instead
of the default. However if the user supplies the value "on" for the path
then the default path will be used for the UART.
tests: Add a very basic functional test of the UART

Use the bootloader provided by Atmel in their AVR2054 App Note to
provide a simple functional test of the UART. This also wires that test
up into the makefiles and into Travis so we can have a simple test
validate that drumfish UARTs still work on each commit.