Update README.md

Add missing prerequisite libboost-regex-dev

Signed-off-by: Katharina <KatCe@users.noreply.github.com>

Author: KatCe

README.md

@@ -74,7 +74,7 @@ Build Steps
 We assume that the RISCV environment variable is set to the RISC-V tools
 install path.
 
-    $ apt-get install device-tree-compiler
+    $ apt-get install device-tree-compiler libboost-regex-dev
     $ mkdir build
     $ cd build
     $ ../configure --prefix=$RISCV