gnu: u-boot: Update to 2020.01.

gnu: u-boot: Update to 2020.01.

* gnu/packages/bootloaders (u-boot): Update to 2020.01.
  [source]: Remove patch u-boot-fix-mkimage-header-verification.
  [native-inputs]: Update to use python 3 and add perl.
  (u-boot-tools)[configure]: Use tools-only_defconfig.
  [arguments]: Update phase updating python-coverage to work with python 3.
  Drop obsolete substitution for test_ofplatdata.
  Disable code coverage tests and binman tests.
* gnu/packages/patches/u-boot-fix-mkimage-header-verification.patch: Remove file.
* gnu/local.mk [dist_patch_DATA]: Update accordingly.