binman: Handle repeated bytes for Python 3

binman: Handle repeated bytes for Python 3

The method of multiplying a character by a number works well for creating
a repeated string in Python 2. But in Python 3 we need to use bytes()
instead, to avoid unicode problems, since 'bytes' is no-longer just an
alias of 'str'.

Create a function to handle this detail and call it from the relevant
places in binman.

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