ipynb: fix LISA_HOME identification if realpath is not available

This patch makes the identification of the LISA installation folder
more robust by depending only on standard shell commands.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
diff --git a/ipynb/ipyserver_start b/ipynb/ipyserver_start
index 7c754d4..60ff7de 100755
--- a/ipynb/ipyserver_start
+++ b/ipynb/ipyserver_start
@@ -20,9 +20,12 @@
     exit 1
 fi
 
-LISA_HOME=$(echo $(realpath $0) | sed 's|/ipynb/ipyserver_start||')
-PYDIR=$LISA_HOME/ipynb
-LOGFILE=$PYDIR/server.log
+# Get base installation path of LISA
+cd "$(dirname $0)/.." &>/dev/null
+LISA_HOME="$(pwd)"
+cd - $>/dev/null
+PYDIR="$LISA_HOME/ipynb"
+LOGFILE="$PYDIR/server.log"
 
 export PYTHONPATH=''
 export PYTHONPATH=$LISA_HOME/libs/utils:$PYTHONPATH