riscv: add firmware partitions to GENERICSD.conf

Create two partitions in the existing space at the beginning of the
image (8MB). These are intended to hold u-boot SPL and u-boot proper.

The partition types selected are compatible with SiFive boards, e.g. the
HiFive Unmatched. They can easily be overridden for a platform that uses
the same scheme but different partition types, e.g. the StarFive
VisionFive v2. Firmware is not actually installed, this too is left for
the user.

It is not as simple to create the firmware partitions after the fact,
e.g. with partition indices 3 and 4. It is a shortcoming of current day
u-boot that the SPL loader looks for a specific partition index, rather
than the partition type, meaning that we will fail to boot if partition
2 doesn't contain u-boot.

Thus, our GENERICSD images become more generically usable with current
RISC-V hardware/firmware platforms.

Reviewed by: manu, karels, imp
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43849

Increase the size of riscv GENERICSD images to 6 GB

The stable/13 snapshot this week failed to build the riscv GENERICSD
image because it ran out of space. Checking main and stable/14
snapshots, they are also low on space, around 100% or more of
capacity. Increase them all from 5 GB to 6 GB. Note, this is the
only riscv image configuration.

Discussed with: cperciva

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

release: increase IMAGE_SIZE for arm, arm64, riscv

Related to: PR 264032
MFC after: 5 minutes
Sponsored by: Rubicon Communications, LLC ("Netgate")

release: riscv64 - increase GENERICSD image size

Sponsored by: Rubicon Communications, LLC ("Netgate")

release: permanently remove the 'reldoc' target and associates

Following 7b1d1a1658ffb69eff93afc713f9e88ed8b20eac, the structure
for the reldoc target has significantly changed as result of the
ASCIIDoctor/Hugo migration. As the release notes related files
on the installation medium are inherently out of date, purge them
entirely.

Discussed within: re, doceng
No objection: re (silence), doceng (silence)
Timeout: 2 weeks
MFC after: 1 week
MFC to: stable/13, stable/12, and stable/11 only
Sponsored by: Rubicon Communications, LLC ("Netgate")

riscv: increase GENERICSD gap

Leave more room for bootloaders at the beginning of the image. In
particular, the u-boot files for the HiFive Unleashed are ~5MB in size.

RISC-V release confs

Add two release flavors for RISC-V. First, the traditional "big-iron"
images, capable of generating distribution sets and VM images. Installer
images won't be built yet, but can be trivially enabled in the future
with the addition of riscv/make-memstick.sh.

Second, a GENERICSD embedded image. I've opted for this instead of
board-specific SD card images as it allows users to just dd the u-boot
they want. The RISC-V hardware ecosystem is still young, so a
configuration for e.g. the new PolarFire SoC Icicle Kit would likely see
very few users.

Reviewed by: gjb
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D27045