rockusb: add read-flash command to dump entire flash to a file

[obbardc]

Read the device's full flash contents to a file, printing the flash size before starting.

Fixes: #26

NOTE: This is a draft as it is currently completely untested as I don't have a rockchip device to hand now. Testing welcome !

[alchark]

@obbardc Rockchip's loader binaries stop returning meaningful data past the first few megabytes, so this option likely requires a huge warning in the console.

Otherwise people might expect it to dutifully dump the whole eMMC with a factory image, which will in fact turn out mostly empty (and the flash already erased - as was the case for me a couple of times using rkdeveloptool)

[sjoerdsimons]

@obbardc Rockchip's loader binaries stop returning meaningful data past the first few megabytes, so this option likely requires a huge warning in the console.

Otherwise people might expect it to dutifully dump the whole eMMC with a factory image, which will in fact turn out mostly empty (and the flash already erased - as was the case for me a couple of times using rkdeveloptool)

This depends on the loader binaries; For iirc RK3399 this worked pretty well and one some RK3328 devices i have it seems to work fine to read from e.g. SD as well. I even have some fun prototypes lying around that expose the devices via NBD and/or UBLK, which can be quite useful.

RK3588 has a buggy loader in this respect and i guess RK3576 is the same here. I can't remember if the upstream u-boot implementation supports these features, but that's something that could get fixed once

[alchark]

Perhaps. I only have RK3588 and RK3576 at hand, and they are definitely quirky, so it sets a dangerous precedent:)

[obbardc]

I wonder if there's any way to detect this bug at runtime and warn the user ? :-)