EC2: Flavour existing AMIs as "base"

Using the recently-added "cloudware flavours" mechanism, turn the
existing EC2 AMIs into a new "base" flavour. The only user-visible
change is that AMI names now include the word "base".

Approved by: re (gjb)
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41791

(cherry picked from commit 7494fb6b7d809257d4eb9eff06ce226143f27add)
(cherry picked from commit a18905a9bfccc962f67b8a6067e7cbd4eb2bba17)