reformat

only cleanup, no functional change
reformat header

only reformat, no functional changes
use _BV macro

while (1<<..) and _BV(..) are equivalent and need the same space, _BV
hides all the shifteroo.
move defines

the USB_SERIAL_INTERNAL is a give away. Other code has no buisiness
with it, move it to the .c to confine it there.
remove functions

we don't need the serial status functions, remove them.
Short circuit access to the control lines.
restructure serial_write a little

let the preprocessor help, add a disabled asm version
add helper to send strings over serial

add convinience functions to send a typical C-string over the usb serial
link.
They should copy the data "in one go", at least more "one go" then
helper which call usb_serial_putchar for every character.
Merge pull request #2 from kaffeemonster/usb_lowlat

Usb lowlat
- Added bit position computation for a given sx channel
- Small typo fix
Added function to read a channel byte
Fixed variable for counting the transferred bits
Smal fixes and bugs
Got sx_get_channel(uint8_t channel) to work