boot: share combine_initrds() between stub/boot

We'd like to use combine_initrds() later in systemd-boot, hence move it
out of stub.c and into shared code.

src/boot/initrd.c

@@ -215,3 +215,48 @@ EFI_STATUS initrd_read_previous(struct iovec *ret_initrd) {
 
         return EFI_SUCCESS;
 }
+
+EFI_STATUS combine_initrds(
+                const struct iovec initrds[], size_t n_initrds,
+                Pages *ret_initrd_pages, size_t *ret_initrd_size) {
+
+        size_t n = 0;
+
+        /* Combine initrds by concatenation in memory */
+
+        assert(initrds || n_initrds == 0);
+        assert(ret_initrd_pages);
+        assert(ret_initrd_size);
+
+        FOREACH_ARRAY(i, initrds, n_initrds) {
+                /* some initrds (the ones from UKI sections) need padding, pad all to be safe */
+                size_t initrd_size = ALIGN4(i->iov_len);
+                if (n > SIZE_MAX - initrd_size)
+                        return EFI_OUT_OF_RESOURCES;
+
+                n += initrd_size;
+        }
+
+        _cleanup_pages_ Pages pages = xmalloc_initrd_pages(n);
+        uint8_t *p = PHYSICAL_ADDRESS_TO_POINTER(pages.addr);
+
+        FOREACH_ARRAY(i, initrds, n_initrds) {
+                size_t pad;
+
+                p = mempcpy(p, i->iov_base, i->iov_len);
+
+                pad = ALIGN4(i->iov_len) - i->iov_len;
+                if (pad == 0)
+                        continue;
+
+                memzero(p, pad);
+                p += pad;
+        }
+
+        assert(PHYSICAL_ADDRESS_TO_POINTER(pages.addr + n) == p);
+
+        *ret_initrd_pages = TAKE_STRUCT(pages);
+        *ret_initrd_size = n;
+
+        return EFI_SUCCESS;
+}

src/boot/initrd.h

@@ -2,6 +2,8 @@
 #pragma once
 
 #include "efi.h"
+#include "iovec-util-fundamental.h"
+#include "util.h"
 
 EFI_STATUS initrd_register(
                 const struct iovec *initrd,
@@ -15,3 +17,5 @@ static inline void cleanup_initrd(EFI_HANDLE *initrd_handle) {
 }
 
 EFI_STATUS initrd_read_previous(struct iovec *ret_initrd);
+
+EFI_STATUS combine_initrds(const struct iovec initrds[], size_t n_initrds, Pages *ret_initrd_pages, size_t *ret_initrd_size);

src/boot/stub.c

@@ -102,50 +102,6 @@ static void combine_measured_flag(int *value, int measured) {
         *value = *value < 0 ? measured : *value && measured;
 }
 
-/* Combine initrds by concatenation in memory */
-static EFI_STATUS combine_initrds(
-                const struct iovec initrds[], size_t n_initrds,
-                Pages *ret_initrd_pages, size_t *ret_initrd_size) {
-
-        size_t n = 0;
-
-        assert(initrds || n_initrds == 0);
-        assert(ret_initrd_pages);
-        assert(ret_initrd_size);
-
-        FOREACH_ARRAY(i, initrds, n_initrds) {
-                /* some initrds (the ones from UKI sections) need padding, pad all to be safe */
-                size_t initrd_size = ALIGN4(i->iov_len);
-                if (n > SIZE_MAX - initrd_size)
-                        return EFI_OUT_OF_RESOURCES;
-
-                n += initrd_size;
-        }
-
-        _cleanup_pages_ Pages pages = xmalloc_initrd_pages(n);
-        uint8_t *p = PHYSICAL_ADDRESS_TO_POINTER(pages.addr);
-
-        FOREACH_ARRAY(i, initrds, n_initrds) {
-                size_t pad;
-
-                p = mempcpy(p, i->iov_base, i->iov_len);
-
-                pad = ALIGN4(i->iov_len) - i->iov_len;
-                if (pad == 0)
-                        continue;
-
-                memzero(p, pad);
-                p += pad;
-        }
-
-        assert(PHYSICAL_ADDRESS_TO_POINTER(pages.addr + n) == p);
-
-        *ret_initrd_pages = TAKE_STRUCT(pages);
-        *ret_initrd_size = n;
-
-        return EFI_SUCCESS;
-}
-
 static void export_stub_variables(EFI_LOADED_IMAGE_PROTOCOL *loaded_image, unsigned profile) {
         static const uint64_t stub_features =
                 EFI_STUB_FEATURE_REPORT_BOOT_PARTITION |    /* We set LoaderDevicePartUUID */