sys: Remove $FreeBSD$: one-line sh pattern

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

Fix some modules to export more used symbols

and remove non-present symbols that are now reported by kmod_syms.awk.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32878

Revert r344675

It's an incorrect approach to solve the problem. We already have a
fdt/fdt_slicer module, it just needs to be wired into the build.

GEOM: Add fdt_slicer to the GEOM flashmap module for fdt-based platforms

geom_flashmap depends on a slicer being available in order to do any
work. On fdt platforms this is provided by fdt_slicer, but this needs
to be available. Often it's compiled into the kernel for platforms that
boot from the relevant media, but this is not always the case. Add the
file to the geom_flashmap module so that it can be used on platforms
which don't always need this functionality available.

sys/modules: normalize .CURDIR-relative paths to SRCTOP

This simplifies make output/logic

Tested with: `cd sys/modules; make ALL_MODULES=` on amd64
MFC after: 1 month
Sponsored by: Dell EMC Isilon

- Allow different slicers for different flash types to be registered
with geom_flashmap(4) and teach it about MMC for slicing enhanced
user data area partitions. The FDT slicer still is the default for
CFI, NAND and SPI flash on FDT-enabled platforms.
- In addition to a device_t, also pass the name of the GEOM provider
in question to the slicers as a single device may provide more than
provider.
- Build a geom_flashmap.ko.
- Use MODULE_VERSION() so other modules can depend on geom_flashmap(4).
- Remove redundant/superfluous GEOM routines that either do nothing
or provide/just call default GEOM (slice) functionality.
- Trim/adjust includes

Submitted by: jhibbits (RouterBoard bits)
Reviewed by: jhibbits