Allow building makefs(8) from another Makefile (such as one in
a seperate directory hierarchy used to build tools). This boils
down to replacing the use of ${.CURDIR} with either ${SRCDIR}
or ${SRCTOP}. SRCDIR is defined as the directory in which the
Makefile lives that bmake(1) is currently reading. Use SRCTOP
when reaching outside of makefs's directory.