build-appliance-image: Fix permissions

build-appliance-image: Fix permissions

Explicitly set all permissions.
This fixes various problems such as:

Unable to create '/home/builder/poky/bitbake/lib/bb/pysh/pyshtables.py'
[Errno 22] invalid mode ('w') or filename '/home/builder/poky/bitbake/lib/bb/pysh/pyshtables.py'

and failure to create sub-directories in the directory 'poky':

mkdir: cannot create directory '<dir>': Invalid argument

While in there, added a few non-functional changes:
Added sample proxy settings into .bashrc
Added environmental variable with SRCREV actually used to populate the Build
Appliance into .bashrc (BA_SRCREV)

[YOCTO#9314]

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>