Update apexer_test the hashtree hash algorithm to sha256

Both apexer and apexer_test apply sha256 to be algorithm of the hashtree.
This patch updates the command line 'avbtool' to apply sha256 in
apexer_test.py.

Fixes: 157447952
Test: atest --host apexer_test
Change-Id: I5e4986e0bc54cb87d9ba5cb8fb9b545d09dc1c1d
1 file changed