[aml] Restore BR2_PACKAGE_UTIL_LINUX_MOUNT

Changeset 81f69d7c8f59922146a0951a114b3ce1c73723eb incorrectly removed BR2_PACKAGE_UTIL_LINUX_MOUNT.  We also need to define BR2_PACKAGE_UTIL_LINUX_BINARIES for it to be selectable.
[aml] Restore BR2_EXT_TOOL_DIRS_EXTRA functionality

Changeset 81565752f52bf5f7a9db85e639a33ee126a74082 incorrectly removed the BR2_EXT_TOOL_DIRS_EXTRA functionality, which meant that /usr/lib/gconv and /usr/share/zoneinfo were not being copied to the target.
[aml] Define BR2_PACKAGE_TZDATA

Newer versions of buildroot provide the tzdata package to copy the time zone data, so we don't need to use BR2_EXT_TOOL_DIRS_EXTRA to copy /usr/share/zoneinfo