package/systemd: fix systemd-boot installation

When the meson build directory was changed to buildroot-build in
4e95062f820a64ecd2402115ee552dbcf2874f02 we forgot to update the
install path for the systemd-boot efi binary.

Fixes:
/usr/bin/install: cannot stat '/home/buildroot/buildroot/output/build/systemd-256.7/build/src/boot/efi/systemd-bootx64.efi': No such file or directory

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/systemd/systemd.mk

@@ -646,7 +646,7 @@ 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 $(@D)/build/src/boot/efi/systemd-boot$(SYSTEMD_BOOT_EFI_ARCH).efi \
+	$(INSTALL) -D -m 0644 $(@D)/buildroot-build/src/boot/efi/systemd-boot$(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