Implement busio.I2c.

* Added asf4_conf/samd*/hpl_sercom_config.h
* Adjusted clocks in peripheral_clk_config.h.
* Put some frozen libs back in CPX for testing.
* Implement common-hal I2C
* Add samd*_peripherals.h in parallel with samd*_pins.h for common
  functions and data.
* Store SERCOM index in pins table for convenience.
* Canonicalize some #include guard names in various .h files.

simpler reset of SERCOMs; remove unused routine

.gitignore

@@ -48,3 +48,12 @@ _build
 # Generated rst files
 ######################
 genrst/
+
+# ctags and similar
+###################
+TAGS
+
+# Merge leftovers
+#################
+*.orig
+

ports/atmel-samd/Makefile

@@ -169,6 +169,8 @@ SRC_ASF := \
 	hal/src/hal_atomic.c \
 	hal/src/hal_delay.c \
 	hal/src/hal_flash.c \
+	hal/src/hal_i2c_m_sync.c \
+	hal/src/hal_io.c \
 	hal/src/hal_sleep.c \
 	hal/src/hal_timer.c \
 	hal/src/hal_usb_device.c \
@@ -177,6 +179,7 @@ SRC_ASF := \
 	hpl/nvmctrl/hpl_nvmctrl.c \
 	hpl/pm/hpl_pm.c \
 	hpl/rtc/hpl_rtc.c \
+	hpl/sercom/hpl_sercom.c \
 	hpl/systick/hpl_systick.c \
 	hpl/tc/hpl_tc.c \
 	hpl/usb/hpl_usb.c \
@@ -213,6 +216,7 @@ SRC_C = \
 	flash_api.c \
 	mphalport.c \
 	reset.c \
+	$(CHIP_FAMILY)_peripherals.c \
 	$(CHIP_FAMILY)_pins.c \
 	tick.c \
 	usb.c \
@@ -246,23 +250,23 @@ endif
 
 SRC_COMMON_HAL = \
 	board/__init__.c \
+	busio/__init__.c \
+	busio/I2C.c \
 	digitalio/__init__.c \
 	digitalio/DigitalInOut.c \
 	microcontroller/__init__.c \
 	microcontroller/Pin.c \
 	microcontroller/Processor.c \
 	neopixel_write/__init__.c \
 	os/__init__.c \
-	time/__init__.c
+	time/__init__.c \
 #	analogio/__init__.c \
 	analogio/AnalogIn.c \
 	analogio/AnalogOut.c \
 	audiobusio/__init__.c \
 	audiobusio/PDMIn.c \
 	audioio/__init__.c \
 	audioio/AudioOut.c \
-	busio/__init__.c \
-	busio/I2C.c \
 	busio/SPI.c \
 	busio/UART.c \
 	neopixel_write/__init__.c \