buildroot

Makefile: add check of binaries architecture

As shown recently by the firejail example, it is easy to miss that a
package builds and installs binaries without actually cross-compiling
them: they are built for the host architecture instead of the target
architecture.

This commit adds a small helper script, check-bin-arch, called as a
GLOBAL_INSTRUMENTATION_HOOKS at the end of the target installation of
each package, to verify that the files installed by this package have
been built for the correct architecture.

Being called as a GLOBAL_INSTRUMENTATION_HOOKS allows the build to error
out right after the installation of the faulty package, and therefore
get autobuilder error detection properly assigned to this specific
package.

Example output with the firejail package enabled, when building for an
ARM target:

ERROR: architecture for ./usr/lib/firejail/libconnect.so is Advanced Micro Devices X86-64, should be ARM
ERROR: architecture for ./usr/bin/firejail is Advanced Micro Devices X86-64, should be ARM
ERROR: architecture for ./usr/lib/firejail/libtrace.so is Advanced Micro Devices X86-64, should be ARM
ERROR: architecture for ./usr/lib/firejail/libtracelog.so is Advanced Micro Devices X86-64, should be ARM
ERROR: architecture for ./usr/lib/firejail/ftee is Advanced Micro Devices X86-64, should be ARM
ERROR: architecture for ./usr/lib/firejail/faudit is Advanced Micro Devices X86-64, should be ARM
ERROR: architecture for ./usr/bin/firemon is Advanced Micro Devices X86-64, should be ARM
ERROR: architecture for ./usr/bin/firecfg is Advanced Micro Devices X86-64, should be ARM

Many thanks to Yann E. Morin and Arnout Vandecappelle for their reviews
and suggestions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>