target-finalize: remove extra /usr/libexec/{*.a,*.la} files

Some packages, sudo for instance, install .a and .la files in
$(TARGET_DIR)/usr/libexec. These files are not needed on target.

This patch refactors the existing "find" invocations in
target-finalize into a single one removing all .a and .la files from
lib, usr/lib and usr/libexec.

[Thomas: rework to use a single "find" invocation, and adjusted the
commit log accordingly.]

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