machine: remove bytes package dependency in flash code

This also moves flash padding code to a single place, since it was
copied 5 times.

This change is necessary in Go 1.24 to avoid an import cycle.

src/machine/machine_nrf.go

@@ -3,7 +3,6 @@
 package machine
 
 import (
-	"bytes"
 	"device/nrf"
 	"internal/binary"
 	"runtime/interrupt"
@@ -386,7 +385,7 @@ func (f flashBlockDevice) WriteAt(p []byte, off int64) (n int, err error) {
 	}
 
 	address := FlashDataStart() + uintptr(off)
-	padded := f.pad(p)
+	padded := flashPad(p, int(f.WriteBlockSize()))
 
 	waitWhileFlashBusy()
 
@@ -444,17 +443,6 @@ func (f flashBlockDevice) EraseBlocks(start, len int64) error {
 	return nil
 }
 
-// pad data if needed so it is long enough for correct byte alignment on writes.
-func (f flashBlockDevice) pad(p []byte) []byte {
-	overflow := int64(len(p)) % f.WriteBlockSize()
-	if overflow == 0 {
-		return p
-	}
-
-	padding := bytes.Repeat([]byte{0xff}, int(f.WriteBlockSize()-overflow))
-	return append(p, padding...)
-}
-
 func waitWhileFlashBusy() {
 	for nrf.NVMC.GetREADY() != nrf.NVMC_READY_READY_Ready {
 	}