Update description related to ARM rootfs (dotnet/coreclr#8962)

Update description related to ARM rootfs (dotnet/coreclr#8962)

Update description in script and documentation related to ARM rootfs.

1. Change arm-softfp to armel to apply recent changes.

2. Change UbuntuCodeName to LinuxCodeName
For arm and armel, rootfs are available from three different Linux distro.
    For arm, Ubuntu is available.
    For armel, Debian and Tizen are available.

Signed-off-by: Hyung-Kyu Choi <hk0110.choi@samsung.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/c73f3a1a5b2616d039a9bf69803fe9942976410e