chromeos: configuring boot environment variables.

Make the boot environment more friendly for my system.

Becasue we do not contain ec in our u-boot image such as google officially
does, it is not possible to "saveenv" into EEPROM. Instead, we put our
environment on boot partition and load them when u-boot starts.
Add "manual_import" environment variable.

More convenient if we would like to load different kernel for test.
Add build command in "pkg" script.
Add some files to be ignored by git.