Add mkimg to the list of bootstrap tools

Having this as part of the bootstrap tools is useful to build disk images
to boot in QEMU (especially when building on a Linux/macOS host where
mkimg is not available). We have been bootstrapping mkimg in CheriBSD for
a long time (using LOCAL_XTOOL_DIRS) but I believe this is also useful
upstream.

Reviewed By:	emaste, brooks
Differential Revision: https://reviews.freebsd.org/D27602

Makefile.inc1

@@ -2287,6 +2287,14 @@ _bootstrap_tools_links+=kbdcontrol
 .endif
 .endif
 
+# 1300102: VHDX support
+.if ${BOOTSTRAPPING} < 1201520 || \
+	(${BOOTSTRAPPING} > 1300000 && ${BOOTSTRAPPING} < 1300102)
+_mkimg=	usr.bin/mkimg
+.else
+_bootstrap_tools_links+=mkimg
+.endif
+
 _yacc=		usr.bin/yacc
 
 .if ${MK_BSNMP} != "no"
@@ -2462,6 +2470,7 @@ bootstrap-tools: ${_bt}-links .PHONY
     ${_flua} \
     ${_crunchide} \
     ${_crunchgen} \
+    ${_mkimg} \
     ${_nmtree} \
     ${_vtfontcvt} \
     ${_localedef} \