build: Correct module dep graph checking and INSTALL_MODULES

[kpfleming]

This patch makes two sets of changes:

• Converts INSTALL_MODULES into a proper array so that it will be processed properly

• Changes syntax in check_dependencies to properly convert grep output into an array (mapfile consumes lines, but in this situation grep outputs space-separated words), and to use canonical syntax for appending to arrays

With these changes the btrfs and sg modules actually get included into the installer image as they should.

[FooDeas]

Could you fix the Shellcheck issue SC2006 in line 71? (see Travis CI details)

[kpfleming]

Done.