PWM implementation uses a timer and interrupts (FRC1), taken from
Espressif's/NodeMCU's implementation and adapted for our use.
8 channels are supported, on pins 0, 2, 4, 5, 12, 13, 14, 15.
Usage:
import machine
pwm0 = machine.PWM(machine.Pin(0))
pwm0.freq(1000)
pwm0.duty(500)
Frequency is shared (ie the same) for all channels. Frequency is
between 1 and 1000. Duty is between 0 and 1023.
1. Updated linker script, now user app appears to contain exception vector
table and oesn't work (faults) without it.
2. Commened out support for GPIO pulldown, which was removed in this SDK
version without clear explanation, but apparently because it was released
without proper validation, and now turns out it doesn't work as expected,
or there's a different function there.
Some rodata items can go in iram/irom segment, but not others. With
this patch ESP now has 24256 bytes of heap ram. It passes 228 out of
248 tests from tests/basics directory.
