- Copy stable/10@285827 to releng/10.2 in preparation for 10.2-RC1
builds.
- Update newvers.sh to reflect RC1.
- Update __FreeBSD_version to reflect 10.2.
- Update default pkg(8) configuration to use the quarterly branch.[1]

Discussed with: re, portmgr [1]
Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r284780, r284811, r284812:

r284780:
Ensure ASSUME_ALWAYS_YES is set when deleting pkg(8) from the
DESTDIR, otherwise it can get stuck in ttyin.

r284811:
Consider PRERELEASE builds snapshots, in addition to CURRENT
and STABLE.

r284812:
Change the hour:minute delimiter from ':' to '-', since
the former is an invalid character in EC2 images.

Approved by: re (glebius, insta-MFC)
Sponsored by: The FreeBSD Foundation

MFC r280299, r280840, r280846, r280879, r280881, r280884, r280928,
r281783, r281809, r281832:

r280299 (cperciva):
When creating VM images, copy the contents of the created filesystem
into a new filesystem before packaging it into a disk image. This
prevents "remnants" of deleted files from showing up in the VM images,
and reduces their compressed size (by about 10% for the cloudware
images) as a result.

r280840 (cperciva):
Clean up filesystem unmounting in vmimage builds:
- Remove vm_umount_base function which is currently unused.
- Add umount_loop function which loops attempting to unmount one
filesystem.
- Replace calls to umount with calls to umount_loop.
- Don't attempt to unmount ${DESTDIR}/dev if it isn't mounted.

The looping is necessary because sometimes umount fails due to
filesystems being busy. The most common cause of such busyness is
periodic(8) jobs running `find / ...`.

r280846 (cperciva):
Improve check for whether ${DESTDIR}/dev is mounted.

r280879 (cperciva):
Add bits for building EC2 disk images.

r280881 (cperciva):
Partially revert r278118 now that the required logic for deciding
whether freebsd-update can be useful has moved into the
firstboot_freebsd_update script.

r280884 (cperciva):
Fix pkg(8) command line in vm_extra_pre_umount().

r280928 (cperciva):
Add code for creating an EC2 AMI.

r281783:
When building VM disk images, vm_copy_base() uses tar(1) to copy
the userland from one md(4)-mounted filesystem to a clean filesystem
to prevent remnants of files that were added and removed from
resulting in an unclean filesystem. When newfs(8) creates the first
filesystem with journaled soft-updates enabled, the /.sujournal file
in the new filesystem cannot be overwritten by the /.sujournal in the
original filesystem.

To avoid this particular error case, do not enable journaled
soft-updates when creating the md(4)-backed filesystems, and instead
use tunefs(8) to enable journaled soft-updates after the new
filesystem is populated in vm_copy_base().

While here, fix a long standing bug where the build environment /boot
files were used by mkimg(1) when creating the VM disk images by using
the files in .OBJDIR.

r281809:
Simplify variable expansion in attempt to fix the vm-image build.

r281832:
Fix locating the /boot files.

Sponsored by: The FreeBSD Foundation

Add bits for building EC2 disk images. Make logic for uploading these
to create EC2 AMIs will come in a later commit.