Extract the modules bootclasspath handling.

Extracts the modules bootclasspath handling into
a separate utils file, with minor refactoring
(no semantical changes to the behaviour). These utils
could be very useful when using ART chroot mode outside
of Art testsuite.

Test: test-art-target, test-art-host.

Change-Id: I5c03605ac234d15c1f3d9bac342f351abe043368