examples: Extends i2ctest to include a second TWI driver

Adds a commonly used alternative to the Atmel TWI driver. This
particular driver is choosen because it covers a non-interrupt
driven approach to using the TWI module.
Changes to allow for free run in core between cycle timers
 with per interval limiting.

Average cycle times drop by about upwards of 50-60+ cycles per emulated cycle,
	dependant on usage.

sim_avr.h: struct avr_t changed.
	added members run_cycle_count and run_cycle_limit.
	run_cycle_count is number of cycles till next cycle timer.
	run_cycle_limit is maximum number of cycles to run per interval.

sim_core.c: avr_run_one
	* run_one_again label added at top.
	* clause added at end which loops to run_one_again given that the core
		is still in a cpu_Running state, run_cycle_count is greater than
		cycles, and no interrups are pending.

sim_cycle_timers.c:
	* static avr_cycle_timer_return_sleep_run_cycles_limited() added.
		run_cycle_count is bounded to run_cycle_limit.
		returns sleep count unbounded, preserving original behavior.

	* static avr_cycle_timer_reset_sleep_run_cycles_limited() added.
		sets new run_cycle_count based on present list of cycle timers.

	* avr_cycle_timer_reset() changed.
		run_cycle_count and run_cycle_limit is set to default values.

	* avr_cycle_timer_register() changed.
	* avr_cycle_timer_cancel() changed.
	* avr_cycle_timer_process() changed.
		call the relevant function to set/maintain run_cycle_count.

	modified:   simavr/sim/sim_avr.c
	modified:   simavr/sim/sim_avr.h
	modified:   simavr/sim/sim_cycle_timers.c
sim_hex: Only decrement maxlen in read_hex_string when a byte is added to the array

Without this fix, the output to the buffer would be limited to maxlen/2 bytes
instead of maxlen. It is obvious from the implementation that the latter
behavior is the expected one, so the function has been altered to reflect that.
Fixes in avr_timer_reconfigure after refactoring

Several fixes after fcce7868a2fe2ef028b7f6c1741a12380b82a9cc:
- Remove zeroing of mode in which erased mode
  seleced in avr_timer_write
- Fix wrong brackets in ?: in avr_timer_wgm_pwm mode
Enhance emulation of PWM mode

- Clear/set output pin on TOP when OCRnX is in clear/set mode,
  previously it was changed only on TCNT=OCRnX
- Call PWM irq for OCRnB even when TOP is OCRnA
Merge pull request #82 from dougszumski/i2c_test_extension

examples: Extends i2ctest to include a second TWI driver
Merge pull request #83 from bsekisser/master-core-run-many-limited

Changes to allow for free run in core between cycle timers
Merge pull request #92 from cyrozap/master

Fix the issue where Intel HEX files with hex strings longer than 32 bytes would be truncated
Merge pull request #94 from hedrok/master

Timer simulation fixes and improvements
build: re-accelerated the build

Simavr used to build pretty quick, and it's been crawling for a while
now, turns out some of the enhancements we added to the SIMAVR path
detection were being evaluated a LOT.
So went to the makefile and made sure most of the static variables are
evaluated once only.

Signed-off-by: Michel Pollet <buserror@gmail.com>
build: fix build of libsimavr.so

Was being linked with itself. Newer gcc are not amused.

Signed-off-by: Michel Pollet <buserror@gmail.com>
tests: Fix makefile

Was using ${OBJ} before it was declared.

Signed-off-by: Michel Pollet <buserror@gmail.com>