Add support for recording EC2 AMI Ids in SSM

If SSMPREFIX is specified, AMI Ids will be recorded in the SSM
Parameter Store under the name

${SSMPREFIX}/${ARCH}/${FLAVOUR}/${ROOTFS}/${REVISION}/${BRANCH}

where ARCH is "amd64" or "arm64", FLAVOUR is "base" (but may have
other options in the future), ROOTFS is "ufs" (but may have other
options in the future), and REVISION and BRANCH have their normal
meanings.

FreeBSD will be using the public prefix "/aws/service/freebsd",
resulting in SSM Parameter names which look like

/aws/service/freebsd/amd64/base/ufs/14.0/CURRENT

Relnotes: yes
Sponsored by: https://patreon.com/cperciva
MFC after: 2 weeks