mkosi: Mark minimal images as Incremental=relaxed

Building these over and over again is extremely wasteful. Let's just
build them once and not touch them again unless -ff is specified or
their cache is out-of-date, which is exactly what Incremental=relaxed
does.

This means mkosi.extra/ and such changes won't be picked up for these
anymore unless -ff is used, but that slight annoyance is totally
outweighed by the time saved building the images.

mkosi/mkosi.images/minimal-0/mkosi.conf

@@ -9,6 +9,7 @@ SplitArtifacts=yes
 
 [Build]
 Environment=SYSTEMD_REPART_OVERRIDE_FSTYPE=squashfs
+Incremental=relaxed
 
 [Content]
 BaseTrees=%O/minimal-base

mkosi/mkosi.images/minimal-1/mkosi.conf

@@ -9,6 +9,7 @@ SplitArtifacts=yes
 
 [Build]
 Environment=SYSTEMD_REPART_OVERRIDE_FSTYPE=squashfs
+Incremental=relaxed
 
 [Content]
 BaseTrees=%O/minimal-base

mkosi/mkosi.images/minimal-base/mkosi.conf

@@ -5,6 +5,7 @@ Format=directory
 
 [Build]
 Environment=SYSTEMD_REQUIRED_DEPS_ONLY=1
+Incremental=relaxed
 
 [Content]
 Bootable=no

mkosi/mkosi.images/minimal-base/mkosi.conf.d/arch.conf

@@ -10,9 +10,6 @@ Packages=
         iproute
         nmap
 
-VolatilePackages=
-        systemd-libs
-
 RemoveFiles=
         # Arch Linux doesn't split their gcc-libs package so we manually remove
         # unneeded stuff here to make sure it doesn't end up in the image.

mkosi/mkosi.images/minimal-base/mkosi.conf.d/centos-fedora.conf

@@ -11,6 +11,3 @@ Packages=
         iproute
         iproute-tc
         nmap-ncat
-
-VolatilePackages=
-        systemd-libs

mkosi/mkosi.images/minimal-base/mkosi.conf.d/debian-ubuntu.conf

@@ -12,7 +12,3 @@ Packages=
         iproute2
         mount
         ncat
-
-VolatilePackages=
-        libsystemd0
-        libudev1

mkosi/mkosi.images/minimal-base/mkosi.conf.d/opensuse.conf

@@ -15,7 +15,3 @@ Packages=
         patterns-base-minimal_base
         sed
         xz
-
-VolatilePackages=
-        libsystemd0
-        libudev1