check-package: check *.mk for typo in variable

check-package: check *.mk for typo in variable

Warn when a variable is defined in a .mk file and it don't start with
the package name.

This function generates false warnings and the maintenance of the
whitelist can be an extra burden, but it catches some typos really hard
to see:
- POPLER_CONF_OPTS [1]
- BALELD_LICENSE [2]
- DRDB_UTILS_DEPENDENCIES [3]
- PERL_LIBWWW_LICENSE_FILES [4]
- AVRDUDR_LICENSE_FILES [5]
- GST1_PLUGINS_ULGY_HAS_GPL_LICENSE [6]
- ON2_8170_LICENSE [7]
- LIBFDTI_CONF_OPTS [8][9]
- IPSEC_DEPENDENCIES [10]

[1] http://patchwork.ozlabs.org/patch/681533
[2] http://patchwork.ozlabs.org/patch/643293
[3] http://patchwork.ozlabs.org/patch/449589
[4] http://patchwork.ozlabs.org/patch/464545
[5] http://patchwork.ozlabs.org/patch/305060
[6] http://patchwork.ozlabs.org/patch/253089
[7] http://patchwork.ozlabs.org/patch/250523
[8] http://patchwork.ozlabs.org/patch/394125
[9] https://github.com/buildroot/buildroot/commit/fe7a4b524b72bcb448f7e723873d8244620cb2f1
[10] https://github.com/buildroot/buildroot/commit/dff1d590b2a0fadf58b6eed60029b2ecbab7c710

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