Support declarative package installs/uninstalls

Adds a new element, package-installs, that provides an interface for
declarative package installs and uninstalls. Packages to install can be
added to an install.d/package-installs-<element-name> file. The set of
packages listed across such files are installed in a single transaction
at the beginning of install.d.

Prefacing the package name with a "-" indicates that the package should
be uninstalled at the end of the install.d phase. Again, the full set of
uninstalls are done in a single transaction.

An element providing a package-installs file should add package-installs
to its element-deps file.

Change-Id: I5b540388eff1079c8dee933b869463371481152b

elements/package-installs/README.rst

@@ -0,0 +1,8 @@
+The package-installs element allows for a declarative method of installing and
+uninstalling packages for an image build. Adding a file under your elements
+install.d directory called package-installs-<element-name> will cause the list
+of packages in that file to be installed at the beginning of the install.d
+phase.
+
+If the package name in the file starts with a "-", then that package will be
+removed at the end of the install.d phase.

elements/package-installs/install.d/00-package-installs

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -eux
+set -o pipefail
+
+PACKAGES=
+
+for PACKAGEFILE in $(find $(dirname $0) -maxdepth 1 -name "package-installs-*" ); do
+    while read pkg; do
+        if [ ${pkg:0:1} = "-" ]; then
+            pkg=${pkg:1}
+        fi
+        PACKAGES="$PACKAGES $pkg"
+    done < $PACKAGEFILE
+done
+
+install-packages $PACKAGES

elements/package-installs/install.d/99-package-uninstalls

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -eux
+set -o pipefail
+
+PACKAGES=
+
+for PACKAGEFILE in $(find $(dirname $0) -maxdepth 1 -name "package-installs-*" ); do
+    while read pkg; do
+        if [ ! ${pkg:0:1} = "-" ]; then
+            continue
+        fi
+        pkg=${pkg:1}
+        PACKAGES="$PACKAGES $pkg"
+    done < $PACKAGEFILE
+done
+
+install-packages -e $PACKAGES