osbase/build/mkimage/osimage: remove initial slash in EFI paths

Many places where these paths are used don't need the initial slash, and
it's easier to add it than to remove it.

Change-Id: Iec189628556c9aa1c022fdd46a2c9fe57692e0aa
Reviewed-on: https://review.monogon.dev/c/monogon/+/4194
Tested-by: Jenkins CI
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
diff --git a/metropolis/node/core/update/update.go b/metropolis/node/core/update/update.go
index 3d3060d..53bcb96 100644
--- a/metropolis/node/core/update/update.go
+++ b/metropolis/node/core/update/update.go
@@ -505,7 +505,7 @@
 		}
 		switch p := e.FilePath[1].(type) {
 		case efivarfs.FilePath:
-			if string(p) == osimage.EFIPayloadPath {
+			if string(p) == "/"+osimage.EFIPayloadPath {
 				if validBootEntryIdx == -1 {
 					validBootEntryIdx = int(idx)
 				} else {
@@ -541,7 +541,7 @@
 						PartitionUUID: s.ESPPart.ID,
 					},
 				},
-				efivarfs.FilePath(osimage.EFIPayloadPath),
+				efivarfs.FilePath("/" + osimage.EFIPayloadPath),
 			},
 		})
 		if err == nil {