patman: Change the location of patman config file

Move the config file from ~/.config/patman to ~/.patman as it is
more appropriate to have it there. Update the same in the README.

Signed-off-by: Vikram Narayanan <vikram186@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>