docs: update C61 build instructions

C61 build has been split up from the target MACHINE=seco-imx8mm-c61 to
two targets, MACHINE=seco-imx8mm-c61-2gb and MACHINE=seco-imx8mm-c61-4gb,
depending on the memory on the card. Reflect that in the build
instructions.

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

docs/build-flavours/linux-flavour.rst

@@ -25,7 +25,8 @@ Supported machines (default in **bold**):
 * qemuarm
 * qemuarm64
 * seco-intel-b68 (SECO SBC-B68)
-* seco-imx8mm-c61 (SECO SBC-C61)
+* seco-imx8mm-c61-2gb (SECO SBC-C61 2GB DRAM)
+* seco-imx8mm-c61-4gb (SECO SBC-C61 4GB DRAM)
 
 Build steps example:
 

flavours/linux/conf-notes.txt

@@ -15,7 +15,8 @@ Supported machines (first is the default):
 - qemux86-64
 - qemux86
 - seco-intel-b68 (SECO SBC-B68)
-- seco-imx8mm-c61 (SECO SBC-C61)
+- seco-imx8mm-c61-2gb (SECO SBC-C61 2GB DRAM)
+- seco-imx8mm-c61-4gb (SECO SBC-C61 4GB DRAM)
 - raspberrypi4-64
 
 MACHINE variable can be set up in conf/local.conf file under build directory