binman: Read map files as text

binman: Read map files as text

These files are text files so should be read as such, so that
string-equality assertions work as expected.

With this binman tests work correctly on Python 2 and Python 3:

PYTHONPATH=/tmp/b/sandbox_spl/scripts/dtc/pylibfdt \
	python ./tools/binman/binman -t

Change first line of binman.py to end "python3":

PYTHONPATH=~/cosarm/dtc/pylibfdt:tools/patman \
	python3 ./tools/binman/binman -t

Signed-off-by: Simon Glass <sjg@chromium.org>