utils/test-pkg: force checking dependencies

utils/test-pkg: force checking dependencies

Currently, if a user runs "make" while specifying a specific package
(IE: make -p foo),  the Makefile logic skips checking to see if all the
dependencies are selected in the specified packages config file. This behavior
is useful to test simple packages which do not have "complex" dependencies.

However; if a developer uses test-pkg -p ${package_name} to check their package,
the package may pass all the checks, but would have otherwise failed with a
simple "make" because the developer may have failed to add a select line in
packages config file, even if there is a new dependency in the packages
Makefile.

Pass the environment variable "BR_FORCE_CHECK_DEPENDENCIES"  to the Makefile in
the test-pkg script,  and check it's value in the Makefile. If the value is
"YES" force checking for dependency issues.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>