Use anchored filenames and wildcards to make it easier to keep the
.gitignore file up to date with future code changes.
The entries are more precise now:
* ‘git add .’ will not start tracking the src/usbboot binary
* ‘git ls-files -i --exclude-standard’ will not list
xburst_stage1/Makefile and xburst_stage2/Makefile as mistakenly
tracked.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
