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
# Contents for ROMFS
### Prerequisites
+Install genromfs
+On Linux, It can be directly installed from package manager using below command
```bash
sudo apt-get install genromfs
```
+Otherwise, Install genromfs from source.
+
+1. Getting the sources
+```bash
+git clone https://github.com/chexum/genromfs.git
+```
+2. Switch to genromfs 0.5.2 (released) version
+```bash
+cd genromfs
+git checkout 0.5.2 -b 0.5.2
+```
+3. Patch genromfs with below change
+```
+diff --git a/Makefile b/Makefile
+index d278efc..72a4c5c 100644
+--- a/Makefile
++++ b/Makefile
+@@ -16,7 +16,7 @@ FILES = COPYING NEWS ChangeLog Makefile \
+ readme-kernel-patch genrommkdev romfs.txt \
+ checkdist
+
+-prefix = /usr
++prefix = /usr/local
+ bindir = $(prefix)/bin
+ mandir = $(prefix)/man
+```
+4. Build and Install genromfs
+```bash
+make
+sudo make install
+```
+
### Steps
1. Enable ROMFS config through menuconfig at *os* folder.
```bash
