bitbake: siggen: Fix inefficient string concatenationHEADmaster

As discussed in https://stackoverflow.com/a/4435752/1710392 , CPython
has an optimization for statements in the form "a = a + b" or "a += b".
It seems that this line does not get optimized, because it has a form a = a + b + c:
data = data + "./" + f.split("/./")[1]

For that reason, it does a copy of data for each iteration, potentially copying megabytes
of data for each iteration.

Changing this line causes SignatureGeneratorBasic::get_taskhash to take 0.06 seconds
instead of 45 seconds on my test setup where SRC_URI points to a big directory.

Note that PEP8 recommends explicitely not to use this optimization which is specific to CPython:
"do not rely on CPython’s efficient implementation of in-place string concatenation for statements in the form a += b or a = a + b"

However, the PEP8 recommended form using "join()" also does not avoid the copy and takes 45 seconds in my test setup:
data = ''.join((data, "./", f.split("/./")[1]))

I have changed the other lines to also use += for consistency only, however those were in the form a = a + b
and were optimized already.

Co-authored-by: JJ Robertson <jrobertson@snap.com>
(Bitbake rev: 195750f2ca355e29d51219c58ecb2c1d83692717)

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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