package/systemd: fix systemd-boot installation

Following the bump of systemd to 257.5 in commit
7d8d48cd2ba1396ea146e94020939f4dfe08c5cb, the installation of
systemd-boot fails with:

/usr/bin/install -D -m 0644 /home/thomas/projets/buildroot/outputs/3/build/systemd-257.5/buildroot-build/src/boot/efi/systemd-bootx64.efi /home/thomas/projets/buildroot/outputs/3/images/efi-part/EFI/BOOT/bootx64.efi
/usr/bin/install: cannot stat '/home/thomas/projets/buildroot/outputs/3/build/systemd-257.5/buildroot-build/src/boot/efi/systemd-bootx64.efi': No such file or directory

Indeed, the systemd-bootx64.efi binary is no longer produced in
buildroot-build/src/boot/efi/, but in buildroot-build/src/boot/.

No autobuilder failures have been found corresponding to this issue,
but it can be reproduced with:

 BR2_x86_64=y
 BR2_x86_corei7=y
 BR2_TOOLCHAIN_EXTERNAL=y
 BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_X86_64_CORE_I7_GLIBC_BLEEDING_EDGE=y
 BR2_INIT_SYSTEMD=y
 BR2_SYSTEM_BIN_SH_NONE=y
 BR2_SYSTEM_DEFAULT_PATH="/bin:/sbin:/usr/bin:/usr/sbin"
 # BR2_PACKAGE_BUSYBOX is not set
 BR2_PACKAGE_SYSTEMD_BOOT=y
 # BR2_TARGET_ROOTFS_TAR is not set

Most likely this was caused by upstream commit
97318131fd06a5bc35454da81dcbbc84f16d9940, even though we didn't check
if this commit was really the culprit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/systemd/systemd.mk

@@ -649,7 +649,7 @@ SYSTEMD_CONF_OPTS += -Dbootloader=enabled
 
 SYSTEMD_BOOT_EFI_ARCH = $(call qstrip,$(BR2_PACKAGE_SYSTEMD_BOOT_EFI_ARCH))
 define SYSTEMD_INSTALL_BOOT_FILES
-	$(INSTALL) -D -m 0644 $(@D)/buildroot-build/src/boot/efi/systemd-boot$(SYSTEMD_BOOT_EFI_ARCH).efi \
+	$(INSTALL) -D -m 0644 $(@D)/buildroot-build/src/boot/systemd-boot$(SYSTEMD_BOOT_EFI_ARCH).efi \
 		$(BINARIES_DIR)/efi-part/EFI/BOOT/boot$(SYSTEMD_BOOT_EFI_ARCH).efi
 	$(INSTALL) -D -m 0644 $(SYSTEMD_PKGDIR)/boot-files/loader.conf \
 		$(BINARIES_DIR)/efi-part/loader/loader.conf