MFC r320252, r320686, r325769:

MFC r320252, r320686, r325769:
 r320252:
  In release/release.sh:
  - Rename chroot_arm_armv6_build_release() to chroot_arm_build_release()
    and make it hardware agnostic (such as armv6 -vs- armv7 -vs- arm64).
  - Evaluate EMBEDDED_TARGET differently so release/tools/arm.subr can
    be used for arm/armv6 and arm64/aarch64.
  - Update comments and copyright.

  In release/tools/arm.subr:
  - In arm_create_disk(), change the default alignment from 63 to 512k,
    fixing a boot issue on arm64 and EFI. [1]
  - Update comments and copyright.

 r320686:
  Fix the ftp-stage target by loosening the constraints on the TARGET
  and TARGET_ARCH variables.

 r325769:
  Update the GUMSTIX image build to use arm/arm TARGET/TARGET_ARCH.
  Update the TARGET/TARGET_ARCH matching in release/release.sh and
  release/Makefile.mirrors for simplification.

Note: The RPI3.conf addition from r320252 is not included, as it is
not supported on 10-STABLE.  Additionally, arm64/aarch64 changes are
also excluded from this commit.

Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@325899 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f