add menuconfig option to enable SO_RCVBUF
This option is required by Arduino and enables netconn connection to be queried for amount of data available in the rx buffer.
See merge request !372
esp32: ets_update_cpu_frequency should set tick scale for both CPUs
ets_update_cpu_frequency ROM function updates g_ticks_per_us, which is has two copies, one for each CPU.
The APP CPU copy of g_ticks_per_us never got updated, resulting in shorter delays produced by ets_delay_us on the APP CPU.
This MR replaces ROM ets_update_cpu_frequency with a copy in IRAM which updates scaling factors on both of the CPUs.
So now we get expected delays (in microseconds):
```
ets_delay_us core=0 expected=50000 actual=50014
ets_delay_us core=1 expected=50000 actual=50015
vTaskDelay core=0 expected=50000 actual=49428
vTaskDelay core=1 expected=50000 actual=50000
```
Reported on the forum: http://esp32.com/viewtopic.php?f=2&t=713#p3722
See merge request !373
SPI master driver
This merge requests adds an SPI Master driver. This driver is (of course) multithread capable, has device abstraction, has command queueing, DMA support and all other sorts of nice things you'd expect from a SPI driver.
See merge request !332
modify bootloader dram start address, swap app bss and data segments
Modify bootloader dram_seg from address 0x3ffc0000 to 0x3fff0000, len from 0x20000 to 0x10000.
Also put app .data before .bss, to reduce the chance .data collides with ROM bootloader stack.
See merge request !375
This change reduces chances that a large .bss segment will push .data all the way into
0x3ffe1320 — 0x3ffe5320 range where the bootloader stack is, creating a problem when
bootloader will be loading application into memory.
With this change, .data would need to be at least 200k big to cause problems.
Modify bootloader dram_seg from address 0x3ffc0000 to 0x3fff0000, len from
0x20000 to 0x10000. Please be notified that this is just a workaround for
fixing app data overwrite bootloader data issue!
1. add fading functions.
2. clear up ledc.c and ledc.h
3. update api doc.
4. add ledc example
5. edit ledc.rst and add readme for example code.
6. add s_ prefix for static global values.
7. add mutex for fade functions
8. minor changes according to the comments.
