buildhistory.bbclass: Fix build-id.txt growing with each build

:Release Notes:
In commit 03d0f6c3b36ad12a22fe08c745b5c413a9b3c649 I've renamed
build-id to build-id.txt in line which appends buildhistory_get_layers,
but I've forgot to update the filename few lines above, where the file
is overwritten every time to start with only header included.

:Detailed Notes:

:Testing Performed:
Only build tested.

:QA Notes:
No change in image.

:Issues Addressed:
[ES-226] Add "Build Configuration" to build-id.txt

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>

Change-Id: Ic35e969b4cba757f4f1e3570c19a467d51ac2900
Reviewed-on: https://g2g.palm.com/3553
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>