spl: fat/fs: Add option to include/exclude FAT write build in SPL

Most of the time SPL only needs very simple FAT reading, so having
CONFIG_IS_ENABLED(FAT_WRITE) to exclude it from SPL build would help
to save 64KiB default max clustersize from memory.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

common/spl/Kconfig

@@ -404,6 +404,13 @@ config SPL_FS_FAT
 	  filesystem from within SPL. Support for the underlying block
 	  device (e.g. MMC or USB) must be enabled separately.
 
+config SPL_FAT_WRITE
+	bool "Support write for FAT filesystems"
+	help
+	  Enable write support for FAT and VFAT filesystems with SPL.
+	  Support for the underlying block device (e.g. MMC or USB) must be
+	  enabled separately.
+
 config SPL_FPGA_SUPPORT
 	bool "Support FPGAs"
 	help

fs/fat/Makefile

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0+
 #
 
-obj-$(CONFIG_FS_FAT)	:= fat.o
-obj-$(CONFIG_FAT_WRITE):= fat_write.o
+obj-$(CONFIG_$(SPL_)FS_FAT) = fat.o
+obj-$(CONFIG_$(SPL_)FAT_WRITE) = fat_write.o

fs/fat/fat.c

@@ -145,7 +145,8 @@ static void get_name(dir_entry *dirent, char *s_name)
 }
 
 static int flush_dirty_fat_buffer(fsdata *mydata);
-#if !defined(CONFIG_FAT_WRITE)
+
+#if !CONFIG_IS_ENABLED(FAT_WRITE)
 /* Stub for read only operation */
 int flush_dirty_fat_buffer(fsdata *mydata)
 {

fs/fs.c

@@ -168,7 +168,7 @@ static struct fstype_info fstypes[] = {
 		.exists = fat_exists,
 		.size = fat_size,
 		.read = fat_read_file,
-#ifdef CONFIG_FAT_WRITE
+#if CONFIG_IS_ENABLED(FAT_WRITE)
 		.write = file_fat_write,
 		.unlink = fat_unlink,
 		.mkdir = fat_mkdir,