[build] Remove PyYAML dependency

This converts all the YAML files to JSON files, as json parsing is built
into python instead of being a separate library requiring installation.

YAML is a superset of JSON, but putting comments in is not quite as obvious
as it is in yaml.

The following glue was used to convert yaml to json:
python -c 'import sys, yaml, json; json.dump(yaml.load(sys.stdin), sys.stdout, indent=4)' < $1 > $2

Clearly I haven't tested this on every single platform, and this
doesn't address the large blobs of yaml in the lpc4300 scripts directory,
only the cortex NVIC generation process.

I've tested a few IRQ driven example apps, and I've checked the generated
output of some known cases like the LM3s that has explicit gaps, and they are
all generated correctly.

include/libopencm3/lpc17xx/irq.json

@@ -0,0 +1,42 @@
+{
+    "irqs": {
+        "0": "wdt", 
+        "1": "timer0", 
+        "2": "timer1", 
+        "3": "timer2", 
+        "4": "timer3", 
+        "5": "uart0", 
+        "6": "uart1", 
+        "7": "uart2", 
+        "8": "uart3", 
+        "9": "pwm", 
+        "10": "i2c0", 
+        "11": "i2c1", 
+        "12": "i2c2", 
+        "13": "spi", 
+        "14": "ssp0", 
+        "15": "ssp1", 
+        "16": "pll0", 
+        "17": "rtc", 
+        "18": "eint0", 
+        "19": "eint1", 
+        "20": "eint2", 
+        "21": "eint3", 
+        "22": "adc", 
+        "23": "bod", 
+        "24": "usb", 
+        "25": "can", 
+        "26": "gpdma", 
+        "27": "i2s", 
+        "28": "ethernet", 
+        "29": "rit", 
+        "30": "motor_pwm", 
+        "31": "qei", 
+        "32": "pll1", 
+        "33": "usb_act", 
+        "34": "can_act"
+    }, 
+    "partname_humanreadable": "LPC 17xx series", 
+    "partname_doxygen": "LPC17xx", 
+    "includeguard": "LIBOPENCM3_LPC17xx_NVIC_H"
+}

include/libopencm3/lpc17xx/irq.yaml

@@ -1,39 +0,0 @@
-includeguard: LIBOPENCM3_LPC17xx_NVIC_H
-partname_humanreadable: LPC 17xx series
-partname_doxygen: LPC17xx
-irqs:
-  0: wdt
-  1: timer0
-  2: timer1
-  3: timer2
-  4: timer3
-  5: uart0
-  6: uart1
-  7: uart2
-  8: uart3
-  9: pwm
-  10: i2c0
-  11: i2c1
-  12: i2c2
-  13: spi
-  14: ssp0
-  15: ssp1
-  16: pll0
-  17: rtc
-  18: eint0
-  19: eint1
-  20: eint2
-  21: eint3
-  22: adc
-  23: bod
-  24: usb
-  25: can
-  26: gpdma
-  27: i2s
-  28: ethernet
-  29: rit
-  30: motor_pwm
-  31: qei
-  32: pll1
-  33: usb_act
-  34: can_act