Standardize EFI's ESP mount point.

Mount the UEFI ESP on /boot/efi. No current system uses this by default, but
there are many ad-hoc schemes that do this in /efi or /esp or /uefi and adding a
new directory at the top-level would have a much higher likelihood of
collision. Document this in /etc/mtree/BSD.root.mtree and create EFIDIR and
related variables in bsd.own.mk.

Differential Revision: https://reviews.freebsd.org/D21344

etc/mtree/BSD.root.dist

@@ -18,6 +18,8 @@
             rockchip              tags=package=runtime
             ..
         ..
+        efi
+        ..
         firmware
         ..
         lua

share/man/man7/hier.7

@@ -53,6 +53,8 @@ Compiled flattened device tree (FDT) files; see
 .Xr fdt 4
 and
 .Xr dtc 1
+.It Pa efi/
+Mount point for EFI System Partition (ESP) on UEFI systems.
 .It Pa firmware/
 loadable kernel modules containing binary firmware for hardware that needs
 firmware downloaded to it to function

share/mk/bsd.own.mk

@@ -56,6 +56,15 @@
 # KMODMODE	KLD mode. [${BINMODE}]
 #
 #
+# EFIDIR	Base path for the UEFI ESP [/boot/efi]
+#
+# EFIOWN	EFIDIR owner. [root]
+#
+# EFIGRP	EFIDIR group. [wheel]
+#
+# EFIMODE	EFIDIR mode. [555]
+#
+#
 # SHAREDIR	Base path for architecture-independent ascii
 #		text files. [/usr/share]
 #
@@ -169,6 +178,10 @@ DTBODIR?=	/boot/dtb/overlays
 DTBOWN?=	root
 DTBGRP?=	wheel
 DTBMODE?=	444
+EFIDIR?=	/boot/efi
+EFIOWN?=	root
+EFIGRP?=	wheel
+EFIMODE?=	555
 
 # Use make.conf / environment LIBDIR as default if set...
 .if !empty(_PREMK_LIBDIR)