tools/fs: update romfs README to install genromfs from source
tools/fs: update romfs README to install genromfs from source

This patch adds details for installing genromfs from source.
It removes Ubuntu specific installation command and adds
a generic installation method which would work in both
Linux and Mac

Signed-off-by: Manohara HK <02db916c8fbcb3f412fbcb70556ea7b9f661857a@samsung.com>

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