Get rid of all hardcoded speeds in VHDL

[sy2002]

Currently there are a lot of places (e.g. keyboard, UART, debouncers, TIL leds, ...) which rely on some hardcoded 50 MHz clock speed. We need to sift through everything and make generic SYSTEM_SPEED statements and have automatically calculated bit widths of all counters depending on the generics (issue #60). Then we need to have one global variable in env1_globals.vhd where we can conveniently set the speed.

This is a prerequisite for a multitude of things: Coming to a 100 MHz clock speed for the "project retro multimedia machine" in V1.7 and coming to a faster than 100 MHz clock speed in V1.8 or later.

Some original text from the TODO.txt before it was removed due to opening this issue:

* Get rid of hardcoded clock speed dependencies:
  + Have a global constant that reflects the speed of the board and refactor
  all hardcoded frequencies to be dependend formulas from this speed
  (e.g. UART, SD card, VGA, debouncer, SD$TIMEOUT_HIGH, etc.)
  + Have a generator that generates the speeds that certain components need
  as input, e.g. the SD Card component needs currently hardcoded 25 MHz
  + Keyboard: remove hardcoded 18000 constant
  + Monitor: gets hardcoded 10000 clock cycles => 2.000 "nop" loops