use prefix for /etc too in Makefile
imx_sdp: support /etc/imx-loader.d/ as configuration file location

In order to allow default configuration file locations under /etc,
which will allow distribution to create packages of this utility.

However, non standard configuration file location is not yet
supported. This is planned using a command line switch.
imx_usb/imx_uart: new argument parsing, add help text

Implement a more sophisticated argument parsing. The UART variant
checks for mandatory arguments and allows optional arguments as
well. Move the verify to the optional arguments at the beginning.
Add configuration directory parameter for the USB variant and a
option to disable CTS/RTS flow control for the UART variant.

The Job/Work parsing itself is still shared and available at the
end of the arguments.  Multiple Job parsing generally works,
however execution seems to be broken right now.

Also implement a help text outlining the available options and
mandatory arguments (if any).