support/scripts: add check-dotconfig.py

support/scripts: add check-dotconfig.py

For the same reason as for 50b747f212be2c9c0f7cf10c674ed488d042715c,
we need to check if the generated configuration file (.config)
contains all symbols present in the defconfig file.

If not there is an issue with the defconfig.

This script will be used in .gitlab-ci.yml.

Inspired by is_toolchain_usable() function from genrandconfig:
https://git.busybox.net/buildroot/tree/utils/genrandconfig?h=2020.02#n164

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr:
  - strip defconfig lines when reading them
  - use a generator to read the defconfig lines
  - no need to strip() again when building the missing list
  - testing the list directly, not its len()
  - simply sys.exit(1) in the error condition
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>