distro/*: allow /boot to be customized

Since the LVM support was added to all distros, our disk
related code is adaptive, i.e. we will set the correct BLS
and grub2 prefix if there a `boot` partiton is present in
the layout after all customizations happen, which includes
LVMification.
One thing that was not yet fully working was layouts that
do not yet have a `/boot` partition but allow LVMification.
In that case `NewPartitionTable` and if `/boot` was the
first (or only) customization, would LVMify the partition
which in turn would create the `/boot` partition; but after
`newPT.ensureLVM()` the call to `newPT.createFilesystem`
with `/boot` would try to create another `/boot` mountpoint.
In order to deal with this situation correctly we are now
using a two phase approach: 1) enlarge existing mountpoints
and collect new ones. 2) if there are new ones and LMVify
was allowed, switch to LVM layout. Do a second pass and now
create or enlarge existing partitions, handling `/boot` in
the process.

internal/client/compose_test.go

@@ -553,7 +553,7 @@ func TestComposeUnsupportedMountPointV0(t *testing.T) {
 		description="TestComposeUnsupportedMountPointV0"
 		version="0.0.1"
 		[[customizations.filesystem]]
-		mountpoint = "/boot"
+		mountpoint = "/etc"
 		size = 4294967296
 		`
 	resp, err := PostTOMLBlueprintV0(testState.socket, bp)