kbuild: install-extmod-build: Fix when given dir outside the build dir

Commit b5e395653546 ("kbuild: install-extmod-build: Fix build when
specifying KBUILD_OUTPUT") tried to address the "build" variable
expecting a relative path by using `realpath --relative-base=.`, but
this only works when the given directory is below the current directory.
`realpath --relative-to=.` will return a relative path in all cases.

Fixes: b5e395653546 ("kbuild: install-extmod-build: Fix build when specifying KBUILD_OUTPUT")
Signed-off-by: James Le Cuirot <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Nathan Chancellor <[e-mail]>

scripts/package/install-extmod-build

···
 	# Clear VPATH and srcroot because the source files reside in the output
 	# directory.
 	# shellcheck disable=SC2016 # $(MAKE) and $(build) will be expanded by Make
-	"${MAKE}" run-command KBUILD_RUN_COMMAND='+$(MAKE) HOSTCC='"${CC}"' VPATH= srcroot=. $(build)='"$(realpath --relative-base=. "${destdir}")"/scripts
+	"${MAKE}" run-command KBUILD_RUN_COMMAND='+$(MAKE) HOSTCC='"${CC}"' VPATH= srcroot=. $(build)='"$(realpath --relative-to=. "${destdir}")"/scripts
 
 	rm -f "${destdir}/scripts/Kbuild"
 fi