nvmutil makefile: use portable assignments

question mark respects environmental variables

but isn't portable

you can just pass as argument on the command line

question mark is more useful for build systems,
but i'm not really bothered. the old way works.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/Makefile

@@ -2,23 +2,23 @@
 # Copyright (c) 2022,2026 Leah Rowe <leah@libreboot.org>
 # Copyright (c) 2023 Riku Viitanen <riku.viitanen@protonmail.com>
 
-CC?=cc
-HELLCC?=clang
+CC = cc
+HELLCC = clang
 
-CFLAGS?=
-LDFLAGS?=
-DESTDIR?=
-PREFIX?=/usr/local
-INSTALL?=install
+CFLAGS=
+LDFLAGS=
+DESTDIR=
+PREFIX = /usr/local
+INSTALL = install
 
 .SUFFIXES: .c .o
 
-LDIR?=
+LDIR=
 
-PORTABLE?=$(LDIR) $(CFLAGS)
-WARN?=$(PORTABLE) -Wall -Wextra
-STRICT?=$(WARN) -std=c90 -pedantic -Werror
-HELLFLAGS?=$(STRICT) -Weverything
+PORTABLE = $(LDIR) $(CFLAGS)
+WARN = $(PORTABLE) -Wall -Wextra
+STRICT = $(WARN) -std=c90 -pedantic -Werror
+HELLFLAGS = $(STRICT) -Weverything
 
 PROG = nvmutil
 
@@ -33,7 +33,7 @@ OBJS = $(SRCS:.c=.o)
 
 # default mode
 
-MODE?=portable
+MODE = portable
 
 # default mode, options