Add todo

TODO.md

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+# OVERLAYS
+1. copy the `*.dtbo`` files in the overlay directory into the boot partition
+2. add two config files into the boot partion, let's call them `sensing.txt`
+   and `dualeth.txt`
+Contents:
+`sensing.txt`
+```
+dtparam=i2c_arm=on
+enable_uart=1
+
+dtoverlay=ed-sdhost
+dtoverlay=i2c-rtc,pcf8563
+
+dtoverlay=spi1-1cs
+dtparam=cs0_pin=18,cs0_spidev=disabled
+dtoverlay=ed-mcp2515-spi1-can0
+dtoverlay=uart2
+dtoverlay=uart3
+dtoverlay=uart4
+dtoverlay=uart5
+gpio=11=op,dl
+[all]
+
+[cm4]
+otg_mode=1
+```
+
+`dualeth.txt`
+```
+dtparam=i2c_vc=on
+dtoverlay=i2c-rtc,pcf85063a,i2c_csi_dsi
+
+[all]
+
+[cm4]
+otg_mode=1
+```
+
+These config files activate some external devices, mainly the RTC, but I have
+two different board which have use different RTCs. The `dualeth` seems to have
+the same one as the official CM4 IO Board, the `sensing` one has a different
+one. Then I'll just `include` the corresponding file in the `config.txt` after
+flashing the image.
+
+# DRIVERS
+1. driver `i2c-brcmstb` (option `CONFIG_I2C_BRCMSTB`) is missing:
+```
+grep -r CONFIG_I2C_BRCMSTB
+./build_dir/target-aarch64_cortex-a72_musl/linux-bcm27xx_bcm2711/linux-5.10.146/.config:# CONFIG_I2C_BRCMSTB is not set
+./target/linux/bcm27xx/bcm2710/config-5.10:# CONFIG_I2C_BRCMSTB is not set
+./target/linux/bcm27xx/bcm2711/config-5.10:# CONFIG_I2C_BRCMSTB is not set
+./target/linux/bcm27xx/bcm2708/config-5.10:# CONFIG_I2C_BRCMSTB is not set
+./target/linux/bcm27xx/bcm2709/config-5.10:# CONFIG_I2C_BRCMSTB is not set
+```
+
+This is needed for the RTC to work. On Raspberry OS it's inside the kernel and
+this is actually making the RTC i2c available. On openwrt I can't see the I2C
+interface, and I think it's because this driver is missing. I don't care if
+it's inside the kernel or as amodule as long as it's on the image.