How to flexibly choose between using RAM or eMMC for the root filesystem in Buildroot

[LeoWeng]

Here are the settings I've used on ICICLE:
BR2_EXTERNAL=../buildroot-external-microchip/ make icicle_defconfig
BR2_EXTERNAL=../buildroot-external-microchip/ make icicle_rootfs_defconfig

make icicle_defconfig is ram filesystem:
rootfs 761M 95M 666M 13% /
devtmpfs 761M 0 761M 0% /dev
tmpfs 778M 0 778M 0% /dev/shm
tmpfs 778M 40K 778M 1% /tmp
tmpfs 778M 44K 778M 1% /run

make icicle_rootfs_defconfig is ext4 by emmc filesystem:
/dev/root 7.1G 178M 6.5G 3% /
devtmpfs 776M 0 776M 0% /dev
tmpfs 777M 0 777M 0% /dev/shm
tmpfs 311M 1.4M 310M 1% /run
tmpfs 777M 0 777M 0% /tmp

how to build-in dependnce ext4 emmc filesystem on /home like:
rootfs 761M 95M 666M 13% /
devtmpfs 761M 0 761M 0% /dev
tmpfs 778M 0 778M 0% /dev/shm
tmpfs 778M 40K 778M 1% /tmp
tmpfs 778M 44K 778M 1% /run
/dev/root 7.1G 178M 6.5G 3% /home <<-- it's emmc