Makefile: Add missing quotes around sort --field-separator

Busybox sort does not handle --field-separator== , replace this
with --field-separator='=' for maximum compatibility.

Fixes: cc5a490c ("Makefile: Sort u-boot-initial-env output")
Reported-by: Milan P. Stanić <mps@arvanta.net>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>