Catch errors in DIB_INIT_SYSTEM export

When you source a file that just does

 export FOO=$(bar)

you miss any invalid return codes from "bar" (even under -e) because
bash returns the value of the "export", which is 0

On centos-minimal, we stopped bringing in systemd early and this was
causing dib-init-system to not know what init was available.  Since it
did not fail correctly, it lead to confusing errors much later in the
build when service files were not copied correctly.  See also
I24ce648485c3d6f3c27ab8f87a638516b3727017

A dib-lint check is added.  One minor fixup is in 00-set-apt-sources
(this one is less likely to cause problems).  I have run dib-lint over
project-config elements and none use this pattern.

Change-Id: I076c08190d40c315ad6a6d96a3823e9fc52630be

bin/dib-lint

@@ -158,6 +158,15 @@ for i in $(find elements -type f   \
         fi
     fi
 
+    # check for
+    #  export FOO=$(bar)
+    # calls.  These are dangerous, because the export hides the return
+    # code of the $(bar) call.  Split this into 2 lines and -e will
+    # fail on the assignment
+    if grep -q 'export .*\$(' $i; then
+        error "Split export and assignments in $i"
+    fi
+
     # check that sudo calls in phases run outside the chroot look
     # "safe"; meaning that they seem to operate within the chroot
     # somehow.  This is not fool-proof, but catches egregious errors,

elements/apt-sources/test-elements/test-sources/environment.d/00-set-apt-sources

@@ -1 +1,2 @@
-export DIB_APT_SOURCES=$(mktemp)
+DIB_APT_SOURCES=$(mktemp)
+export DIB_APT_SOURCES

elements/dib-init-system/environment.d/10-dib-init-system.bash

@@ -1 +1,2 @@
-export DIB_INIT_SYSTEM=$(PATH="$PATH:$(dirname $BASH_SOURCE)/.." dib-init-system)
+DIB_INIT_SYSTEM=$(PATH="$PATH:$(dirname $BASH_SOURCE)/.." dib-init-system)
+export DIB_INIT_SYSTEM