Fixing issues with variable quoting and arrays

archiso/mkarchiso:
Calls to _pacman() need to be done with multiple parameters (e.g. array) instead of one string, as string splitting is
not done in that function anymore.
Turning _iso_efi_boot_args from string into an array to have an easier time of passing it to xorriso.
Calling xorriso within the if statements instead of providing -quiet via variable.
Fixing command_install() to provide packages separately to _pacman()

configs/releng/build.sh:
Replacing all newlines when retrieving the packages from packages.x86_64 with spaces so they will be properly provided
to "mkarchiso install".