udev-cache: take a read-only rootfs into consideration

In case of a read-only rootfs, we skip the process of generating
udev cache, as the data cannot be persisted between reboots.

However, it's possbile that the $DEVCACHE (default to /etc/dev.tar)
exists in a read-only rootfs, no matter how it's generated or installed.
In such situation, we try to use $DEVCACHE if possible.

Besides the basic changes in the logic of udev cache handling,
this patch also adds code to output more information if the udev
cache is not used and VERBOSE enabled.

This patch also changes the readfile function to readfiles function
so that it could handle more than one file at once.

(From OE-Core rev: 4ec1266e7e1aacdb9d3d0fc5cd6307b60df1731e)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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