Add examples for "make buildenv".

MFC after:	2 weeks
Sponsored by:	DARPA, AFRL

share/man/man7/development.7

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 30, 2018
+.Dd December 6, 2018
 .Dt DEVELOPMENT 7
 .Os
 .Sh NAME
@@ -135,6 +135,29 @@ build has been completed in the past, not on a fresh source tree:
 cd src
 make -sj8 kernel KERNFAST=1
 .Ed
+.Pp
+To rebuild parts of
+.Fx
+for another CPU architecture,
+first prepare your source tree by building the cross-toolchain:
+.Bd -literal -offset indent
+cd src
+make -sj8 toolchain TARGET_ARCH=armv6
+.Ed
+.Pp
+Afterwards, to build and install a single piece of userspace, use:
+.Bd -literal -offset indent
+cd src/bin/ls
+make buildenv TARGET_ARCH=armv6
+make clean all install DESTDIR=/clients/arm
+.Ed
+.Pp
+Likewise, to quickly rebuild and reinstall the kernel, use:
+.Bd -literal -offset indent
+cd src
+make buildenv TARGET_ARCH=armv6
+make -sj8 kernel KERNFAST=1 DESTDIR=/clients/arm
+.Ed
 .Sh SEE ALSO
 .Xr svnlite 1 ,
 .Xr witness 4 ,