build: Correct module dep graph checking and INSTALL_MODULES (#141)
build: Correct module dep graph checking and INSTALL_MODULES (#141)

* build: Correct module dep graph checking and INSTALL_MODULES

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.

* build: Change to modern bash syntax for process substitution

Addresses output from ShellCheck.

FooDeas/raspberrypi-ua-netinst