EC2: Don't enable firstboot_pkgs in small flavour

The EC2 "base" flavour installs the devel/py-awscli package at boot
time by default; we don't do this in the "small" flavour, so the
default behaviour was to update the FreeBSD-ports repository and
then do nothing with it.

Turn off firstboot_pkgs by default; if someone is using the "small"
flavour of AMIs and wants to install packages at instance launch
time, they simply need to add 'firstboot_pkgs_enable="YES"' to
/etc/rc.conf (which they must already be editing via user-data, in
order to provide the list of packages they want installed).

Sponsored by:	Amazon
MFC after:	3 days
MFC to:		stable/15
Relnotes:	EC2 "small" images now have firstboot_pkgs_enable="NO".

release/tools/ec2-small.conf

@@ -33,7 +33,7 @@ export VM_EXTRA_PACKAGES="${VM_EXTRA_PACKAGES} ec2-scripts \
 
 # Services to enable in rc.conf(5).
 export VM_RC_LIST="${VM_RC_LIST} ec2_configinit ec2_ephemeral_swap \
-    ec2_fetchkey ec2_loghostkey firstboot_pkg_upgrade firstboot_pkgs \
+    ec2_fetchkey ec2_loghostkey firstboot_pkg_upgrade \
     growfs sshd"
 
 vm_extra_pre_umount() {