icu: make filtered data generation optional, serial and off by default

icu data generation was found to be racy, and causig AB failures;
making it serial and leaving it on is not an option as it regresses
to several minutes.

The specific bug is that rules.mk has:

LD_LIBRARY_PATH=../lib:../stubdata:../tools/ctestfw:$LD_LIBRARY_PATH  ../bin/gencnval -s . -d ./out/build/icudt66l mappings/convrtrs.txt

which creates a file and numerous rules like

LD_LIBRARY_PATH=../lib:../stubdata:../tools/ctestfw:$LD_LIBRARY_PATH  ../bin/genrb -s ./misc -d ./out/build/icudt67l -i ./out/build/icudt67l -k -q numberingSystems.txt

which quietly read it. There is no prerequisite for the former to complete first.

The race is extra complicated to fix as rules.mk is itself
generated through a custom in-tree python tool.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>