meson: use 0|1 for SD_BOOT

We converted to not using #ifdef for most of our defines because the syntax is
nicer and we are protected against typos and can set -Werror=undef. Let's do
the same for SD_BOOT. The define is nicely hidden in build.h for normal builds,
and for EFI builds we were already setting SD_BOOT on the commandline.

src/boot/efi/efi-string.c

@@ -5,7 +5,7 @@
 
 #include "efi-string.h"
 
-#ifdef SD_BOOT
+#if SD_BOOT
 #  include "util.h"
 #else
 #  include <stdlib.h>
@@ -378,7 +378,7 @@ bool efi_fnmatch(const char16_t *pattern, const char16_t *haystack) {
 DEFINE_PARSE_NUMBER(char, parse_number8);
 DEFINE_PARSE_NUMBER(char16_t, parse_number16);
 
-#ifdef SD_BOOT
+#if SD_BOOT
 /* To provide the actual implementation for these we need to remove the redirection to the builtins. */
 #  undef memcmp
 #  undef memcpy
@@ -414,7 +414,7 @@ _used_ _weak_ void *memcpy(void * restrict dest, const void * restrict src, size
         if (!dest || !src || n == 0)
                 return dest;
 
-#ifdef SD_BOOT
+#if SD_BOOT
         /* The firmware-provided memcpy is likely optimized, so use that. The function is guaranteed to be
          * available by the UEFI spec. We still make it depend on the boot services pointer being set just in
          * case the compiler emits a call before it is available. */
@@ -441,7 +441,7 @@ _used_ _weak_ void *memset(void *p, int c, size_t n) {
         if (!p || n == 0)
                 return p;
 
-#ifdef SD_BOOT
+#if SD_BOOT
         /* See comment in efi_memcpy. Note that the signature has c and n swapped! */
         if (_likely_(BS)) {
                 BS->SetMem(p, n, c);

src/boot/efi/efi-string.h

@@ -109,7 +109,7 @@ bool efi_fnmatch(const char16_t *pattern, const char16_t *haystack);
 bool parse_number8(const char *s, uint64_t *ret_u, const char **ret_tail);
 bool parse_number16(const char16_t *s, uint64_t *ret_u, const char16_t **ret_tail);
 
-#ifdef SD_BOOT
+#if SD_BOOT
 /* The compiler normally has knowledge about standard functions such as memcmp, but this is not the case when
  * compiling with -ffreestanding. By referring to builtins, the compiler can check arguments and do
  * optimizations again. Note that we still need to provide implementations as the compiler is free to not

src/boot/efi/meson.build

@@ -175,7 +175,7 @@ efi_config_h = configure_file(
 
 efi_cflags = [
         '-DGNU_EFI_USE_MS_ABI',
-        '-DSD_BOOT',
+        '-DSD_BOOT=1',
         '-ffreestanding',
         '-fshort-wchar',
         '-fvisibility=hidden',