Added proper option names to the generate_arduino_firmware call.

In generate_avr_firmware the option names for HDRS, LIBS, AFLAGS
got left out. This mixed up all entries for these options and
they ended up in the SRCS option.
Revert "Added proper option names to the generate_arduino_firmware call."

This reverts commit ece86f67a059ba58d02602ae747a912dcdea7a69.
Fixing problem when using HDRS, LIBS, AFLAGS options with generate_avr_firmware

These options might be set or left out (i.e. empty).
So the variables holding the values (e.g. INPUT_HDRS for HDRS)
are checked whether they are empty or not. If they are not
empty the according option name should be added to the front
of the list.

This has been done by setting the variable with a string
containing the option name followed by a space and the
variable content.
The result is not a list with the option name as first
element. Since the space character doesn't work as a
list item delimiter, the first element is a mixture
of option name, space and the old first element in the list.
This didn't get parsed later and resulted in
all these options getting added to the SRCS option.

I changed the update of the lists to use the cmake
list functions. The proper option name gets now inserted into
the lists at the beginning.
Changing the update of the option variables for generate_avr_library

The option lists for HDRS and LIBS in generate_avr_library are
producing mixed up lists if the set function is used with
a space character as delimiter between option name and rest of
list. Using list function for inserting option name helped.
Merge pull request #90 from TheMiles/master

Fixing problems with generate_avr_firmware and generate_avr_library