package/base-files: caldata: work around dd's limitation

package/base-files: caldata: work around dd's limitation

tl;dr: dd will silently truncate the output if reading from special
files (e.g. sysfs attributes) with a too large bs parameter.

This problem was exposed on some RouterBOARD ipq40xx devices which use a
caldata payload which is larger than PAGE_SIZE, contrary to all other
currently supported RouterBOARD devices: the caldata would fail to
properly load with the current scripts.

Background: dd doesn't seem to correctly handle read() results that
return less than requested data. sysfs attributes have a kernel exchange
buffer which is at most PAGE_SIZE big, so only 1 page can be read() at a
time. In this case, if bs is larger than PAGE_SIZE, dd will silently
truncate blocks to PAGE_SIZE. With the current scripts using bs=<size>
count=1, the data is truncated to PAGE_SIZE as soon as the requested
<size> exceeds this value.

This commit works around this problem by using `cat` in the caldata
routines that can read from a file (routines that read from mtd devices
are untouched). cat correctly handles partial read requests. The output
is then piped to dd with the same parameters as before, to ensure that
the resulting file remains exactly the same.

This is a simple workaround, the downside is that it uses a pipe and one
more executable, and therefore has a larger memory footprint and is
slower. This is deemed acceptable considering these routines are only
used at boot time.

Tested-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>