local.conf: update with C61 build change

Update the names used for C61 targets, there are now two instead
of one before.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

flavours/linux/local.conf.sample

@@ -23,7 +23,8 @@
 #MACHINE ?= "qemux86"
 #MACHINE ?= "qemux86-64"
 #MACHINE ?= "seco-intel-b68"
-#MACHINE ?= "seco-imx8mm-c61"
+#MACHINE ?= "seco-imx8mm-c61-2gb"
+#MACHINE ?= "seco-imx8mm-c61-4gb"
 #MACHINE ?= "raspberrypi4-64"
 #
 # This sets the default machine to be qemux86 if no other machine is selected: