Kindle: Find the input device the hard way on the PW5

[NiLuJe]

Fix #11050 (for real this time).

My first PoC unearthed an unfortunate bug in the input backend (koreader/koreader-base#1685), but this one should behave.

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This change is 

[NiLuJe]

The ABS cap check is extremely dumb here, see plugins/externalkeyboard.koplugin/find-keyboard.lua for a more better implementation ;p.

On my own device, the power button doesn't report any EV_ABS capabilities (as it should, it's just a button), so this should be good enough for now.

[NiLuJe]

For context, on affected devices:

ls -lash /sys/class/input

total 0      
     0 drwxr-xr-x    2 root     root           0 Oct 29 18:56 .
     0 drwxr-xr-x   57 root     root           0 Oct 29 18:56 ..
     0 lrwxrwxrwx    1 root     root           0 Oct 29 18:56 event0 -> ../../devices/platform/10019000.i2c/i2c-1/1-004b/gpio-keys.7.auto/input/input0/event0
     0 lrwxrwxrwx    1 root     root           0 Oct 29 18:56 event1 -> ../../devices/virtual/input/input1/event1
     0 lrwxrwxrwx    1 root     root           0 Oct 29 18:56 input0 -> ../../devices/platform/10019000.i2c/i2c-1/1-004b/gpio-keys.7.auto/input/input0
     0 lrwxrwxrwx    1 root     root           0 Oct 29 18:56 input1 -> ../../devices/virtual/input/input1

(Which is slightly different than on my 5.14.1.1 device, but I lost the backlog and the device is stashed now, so you'll have to take my word for it ;p.)

And there's only the power button in /dev/input/by-path.

[hius07]

Confirm, it fixed PW5 G002DK, works well (from 4pda.ru).

[hius07]

But there's another PW5 showing

ffi.load: blitbuffer
./luajit: ./ffi/framebuffer_linux.lua:55: cannot open framebuffer
stack traceback:
	[C]: in function 'assert'
	./ffi/framebuffer_linux.lua:55: in function 'init'
	./ffi/framebuffer_mxcfb.lua:803: in function 'init'
	./ffi/framebuffer.lua:138: in function 'new'
	frontend/device/kindle/device.lua:1203: in function 'init'
	frontend/device.lua:62: in main chunk
	[C]: in function 'require'
	./reader.lua:151: in main chunk
	[C]: at 0x00013ead

[NiLuJe]

@hius07: That one was for an incomplete JB ;).