Implement "ct-ng" building, add "linux-mips".

Add a mechanism to construct a full cross-compiler environment using the
"crosstool-ng" cross-compiler building utility. This is implemented in
the new "common.crosstool" include, and augments the "dockcross/base"
base image.

Update Makefile to consolidate Dockerfile generation, notably the
"sed"-based inclusion directives.

Finally, employ all of this to generate a "linux-mips" 32-bit hard-float
MIPS cross-compiler Dockcross image.

common.crosstool

@@ -0,0 +1,46 @@
+# crosstool.common
+#
+# Common Docker instructions to install "crosstool-ng" and build a full
+# cross-compiler suite from a crosstool-ng configuration, CROSSTOOL_CONFIG.
+#
+# This import complements the "dockcross/base" image, adding:
+# - "ct-ng", a cross-compiler building utilty.
+# - A cross-compiler suite configured in "crosstool-ng.config".
+#
+# The generated cross-compiler will have a CROSS_ROOT of:
+#   ${XCC_PREFIX}/${CROSS_TRIPLE}
+#
+# A given platform will need to supply the appropriate "crosstool-ng.config" to
+# generate its cross-compiler. This can be built using "ct-ng menuconfig" to
+# generate a configuration.
+
+# Install Debian packages required for `ct-ng`.
+RUN apt-get update --yes && apt-get install --no-install-recommends --yes \
+  gawk \
+  gperf \
+  help2man \
+  texinfo \
+&& apt-get clean --yes
+
+ENV XCC_PREFIX=/usr/xcc
+
+# Add the crosstool-ng script and image-specific toolchain configuration into
+# /dockcross/.
+#
+# Afterwards, we will leave the "ct-ng" config in the image as a reference
+# for users.
+COPY \
+  imagefiles/install-crosstool-ng-toolchain.sh \
+  crosstool-ng.config \
+  /dockcross/
+
+# Build and install the toolchain, cleaning up artifacts afterwards.
+RUN mkdir /dockcross/crosstool \
+&& cd /dockcross/crosstool \
+&& /dockcross/install-crosstool-ng-toolchain.sh \
+  -p "${XCC_PREFIX}" \
+  -c /dockcross/crosstool-ng.config \
+&& rm -rf /dockcross/crosstool /dockcross/install-crosstool-ng-toolchain.sh
+
+# Restore our default workdir (from "dockcross/base").
+WORKDIR /work