Updated READMEs in submodules to make it clearer that builds should a…

…lways occur at top-level (#89)

README.md

@@ -22,20 +22,15 @@ sudo apt update
 sudo apt install autoconf automake autotools-dev bc bison build-essential curl \
 expat libexpat1-dev flex gawk gcc git gperf libgmp-dev libmpc-dev libmpfr-dev \
 libtool texinfo tmux patchutils zlib1g-dev wget bzip2 patch vim-common lbzip2 \
-python pkg-config libglib2.0-dev libpixman-1-dev device-tree-compiler
-```
-
-Some of the utilities also use ``expect`` so we recommend that you install that as well though it is not strictly necessary.
-```
-sudo apt install expect
+python pkg-config libglib2.0-dev libpixman-1-dev device-tree-compiler expect
 ```
 
 Setup Tools and Submodules
 ```
 ./fast-setup.sh
 ```
 
-Build All for QEMU
+Build All
 ```
 make
 ```
@@ -60,6 +55,11 @@ Terminate QEMU
 poweroff
 ```
 
+## Rebuilding
+
+Any modifications in a submodule should be built with the top-level
+`make` in Keystone.
+
 # Documentation
 
 See [docs](http://docs.keystone-enclave.org) for detailed