In sys/arch/i386/stand/bootxx/Makefile there is a construction
which attempts to use a common build directory for libkern etc.
This doesn't work as intended:

# make includes
includes ===> bootxx_cd9660
includes ===> bootxx_ffsv1
[...]
mkdir -p /usr/src/sys/arch/i386/stand/bootxx/lib
mkdir: /usr/src/sys/arch/i386/stand/bootxx/lib: Read-only file system
*** Error code 1

The Makefile defines the directory by:
LIBOBJ:= ${.OBJDIR}

If I check this I get:
# make -V .OBJDIR
/usr/src/sys/arch/i386/stand/bootxx/obj.zelz27
# make -V LIBOBJ
/usr/src/sys/arch/i386/stand/bootxx

This looks like make's fault. Or is the Makefile trying something
illegal?
(same problem in sys/arch/i386/stand/boot)

best regards
Matthias