linker: sort app shared mem partition by alignment

If CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT is enabled,
the app shared memory partition may cause waste of memory
due to the need for padding.

For example, tests/subsys/jwt and board mps2_an385:

  z_test_mem_partition: addr 0x20000000, size 52
  z_libc_partition    : addr 0x20000040, size 4
  k_mbedtls_partition : addr 0x20008000, size 32736

    ending at 0x2000ffff, taking up 65536 bytes

With power-of-two size and alignment requirement,
k_mbedtls_partition takes up 32KB memory and needs to be
aligned on 32KB boundary. If the above partitions are
ordered as shown, there needs to be a lot of padding
after z_libc_partition before k_mbedtls_partition can
start. In order to minimize padding, these partitions
need to be sort by size in descending order.

After the changes here,	the partitions are:

  k_mbedtls_partition : addr 0x20000000, size 32736
  z_test_mem_partition: addr 0x20008000, size 52
  z_libc_partition    : addr 0x20008040, size 4

    ending at 0x2000805f, taking up 32864 bytes

With the above example, sorting results in a saving
of 32672 bytes of saving.

Fixes #14121

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

include/linker/app_smem.ld

@@ -1,2 +1,36 @@
-/* space holder  */
-APP_SMEM_SECTION()
+/*
+ * This hackish way of including files is due to CMake issues:
+ *   https://gitlab.kitware.com/cmake/cmake/issues/11985
+ *   https://gitlab.kitware.com/cmake/cmake/issues/13718
+ *
+ * When using the "Unix Makefiles" generator, CMake simply
+ * greps for "#include" to generate dependency list.
+ * So if doing it normally, both files are being included
+ * in the dependency list. This creates weird dependency
+ * issue:
+ *
+ *   1. Using A.ld to create a linker script A.cmd.
+ *   2. Using A.cmd to generate A_prebuilt.elf.
+ *   3. Using A_prebuilt.elf to create B.ld.
+ *   4. Creating B.cmd with B.ld.
+ *   5. Creating B_prebuilt.elf using B.cmd.
+ *
+ * Since the dependency list of A.cmd contains both
+ * A.ld and B.ld, when make is invoked again, B.ld
+ * is newer than A.cmd so everything from this point on
+ * gets rebuilt. In order to break this cycle, this
+ * hackish needs to be used since CMake does not parse
+ * macros, and thus these will not appear in
+ * the dependency list. The dependencies should then be
+ * put in CMakeLists.txt instead.
+ *
+ * Note: Ninja generator does not suffer from this issue.
+ */
+#ifdef LINKER_APP_SMEM_UNALIGNED
+#define APP_SMEM_LD <app_smem_unaligned.ld>
+#else
+#define APP_SMEM_LD <app_smem_aligned.ld>
+#endif
+
+#include APP_SMEM_LD
+#undef APP_SMEM_LD

include/linker/app_smem_aligned.ld

@@ -0,0 +1,2 @@
+/* space holder  */
+APP_SMEM_SECTION()

include/linker/app_smem_unaligned.ld

@@ -0,0 +1,2 @@
+/* space holder  */
+APP_SMEM_SECTION()