udev-cache: replace readfiles() with cmp

Currently, udev-cache system configurations are compared as shell string
variables, read into memory with the readfiles() function. This is more
complex, and significantly (27-41%) slower, than comparing them using
`cmp`. (Performance was verified on both Cortex-A9 and Intel Nehalem
systems.)

So just use cmp. This requires a few other small changes:

exclude /proc/atags from CMP_FILE_LIST if it doesn't exist to avoid
errors in `cat` and `cmp`.

`cmp -q` doesn't exist in busybox, so instead, redirect output to
/dev/null.

(From OE-Core rev: e8ea6a29ed3ab9892a3bc7ee8249f10688c0af29)

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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