Adapt script and instructions for alt config.h

configs/README.txt

@@ -0,0 +1,24 @@
+This directory contains example configuration files.
+
+The examples are generally focused on a particular usage case (eg, support for
+a restricted number of ciphersuites) and aim at minimizing resource usage for
+this target. They can be used as a basis for custom configurations.
+
+These files are complete replacements for the default config.h. To use one of
+them, you can pick one of the following methods:
+
+1. Replace the default file include/polarssl/config.h with the chosen one.
+
+2. Define POLARSSL_CONFIG_FILE and adjust the include path accordingly.
+   For example, using make:
+
+    CFLAGS="-I$PWD/configs -DPOLARSSL_CONFIG_FILE='<foo.h>'" make
+
+   Or, using cmake:
+
+    rm CMakeCache.txt
+    CFLAGS="-I$PWD/configs -DPOLARSSL_CONFIG_FILE='<foo.h>'" cmake .
+    make
+
+Note that the second method also works if you want to keep your custom
+configuration file outside the PolarSSL tree.