check-package: check *.mk files

check-package: check *.mk files

Warn when there are obvious indentation errors:
- the number of expect tabs is not yet checked since it is more complex
  to achieve;
- the content inside define ... endef should be indented with tab(s),
  see [1];
- line just after a backslash should be indented with tab(s), see [2];
- other lines should not be indented, see [3];
- ignore empty lines and comments.
Warn when there is no well-formatted header in the file:
- 80 hashes at lines 1 and 5;
- 1 hash at lines 2 and 4;
- empty line at line 6;
- see [4];
- ignore files that only include other mk files.
Warn when there are more than one space before backslash, see [5].
Warn when there is a trailing backslash [6].
Warn for flags set to default value YES or NO, see [7], [8], [9].

[1] http://patchwork.ozlabs.org/patch/681429/
[2] http://patchwork.ozlabs.org/patch/681430/
[3] http://patchwork.ozlabs.org/patch/559209/
[4] http://nightly.buildroot.org/#writing-rules-mk
[5] http://patchwork.ozlabs.org/patch/649084/
[6] http://patchwork.ozlabs.org/patch/535550/
[7] http://patchwork.ozlabs.org/patch/704718/
[8] http://nightly.buildroot.org/#_infrastructure_for_packages_with_specific_build_systems
[9] http://nightly.buildroot.org/#_infrastructure_for_autotools_based_packages

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>