main/postmarketos-initramfs: add USB booting support

Split from !4430 (closed)

How to boot from USB drives:

1. Make sure the extcon has OTG support and included in initramfs
2. Make sure the PMIC supports the power to OTG devices, and included in initramfs. Otherwise, a hub with external power supply is required.
3. Make sure module usb-storage is enabled in initramfs
4. pmbootstrap install --sdcard=/dev/<usb_drive>
   • pmbootstrap@942ee207 is required to ignore sdcard check. Thanks @ollieparanoid for the patch.
5. pmbootstrap flasher flash_kernel
6. Insert the USB drive to OTG and boot