distro/rhel: append dracut options from installer config to image

The AdditionalDracutModules and AdditionalDrivers were already being set
on each installer image, but let's change it to append in case any
values are set by the initialiser or any other prior code.

pkg/distro/rhel/images.go

@@ -506,8 +506,8 @@ func EdgeInstallerImage(workload workload.Workload,
 	}
 
 	if installerConfig != nil {
-		img.AdditionalDracutModules = installerConfig.AdditionalDracutModules
-		img.AdditionalDrivers = installerConfig.AdditionalDrivers
+		img.AdditionalDracutModules = append(img.AdditionalDracutModules, installerConfig.AdditionalDracutModules...)
+		img.AdditionalDrivers = append(img.AdditionalDrivers, installerConfig.AdditionalDrivers...)
 	}
 
 	instCust, err := customizations.GetInstaller()
@@ -660,7 +660,8 @@ func EdgeSimplifiedInstallerImage(workload workload.Workload,
 	}
 
 	if installerConfig != nil {
-		img.AdditionalDracutModules = installerConfig.AdditionalDracutModules
+		img.AdditionalDracutModules = append(img.AdditionalDracutModules, installerConfig.AdditionalDracutModules...)
+		img.AdditionalDrivers = append(img.AdditionalDrivers, installerConfig.AdditionalDrivers...)
 	}
 
 	return img, nil
@@ -706,8 +707,8 @@ func ImageInstallerImage(workload workload.Workload,
 	}
 
 	if installerConfig != nil {
-		img.AdditionalDracutModules = installerConfig.AdditionalDracutModules
-		img.AdditionalDrivers = installerConfig.AdditionalDrivers
+		img.AdditionalDracutModules = append(img.AdditionalDracutModules, installerConfig.AdditionalDracutModules...)
+		img.AdditionalDrivers = append(img.AdditionalDrivers, installerConfig.AdditionalDrivers...)
 	}
 
 	instCust, err := customizations.GetInstaller()