patman: Handle creation of patman config file

patman shouts when it couldn't find a $(HOME)/.patman file.
Handle it in a sane way by creating a new one for the user.
It looks for a user.name and user.email in the global .gitconfig
file, waits for the user input if it can't find there. Update the
same in the README

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