examples/sotest: fix xxd symbol name mismatch in CMake ROMFS generation

xxd -i derives the generated array/length identifier names from the
exact path string it's given. The CMake build passed the absolute
${SOTEST_ROMFS_IMG} path to xxd, so every non-alphanumeric character
in the build directory path (e.g. every '/') got folded into the
symbol name instead of producing plain 'sotest_romfs_img'/
'sotest_romfs_img_len' - the names sotest_main.c actually declares
extern and links against. This built fine with the Makefile build
(which already only ever passes xxd a bare relative filename) but
failed to link on any CMake-built configuration with
CONFIG_EXAMPLES_SOTEST_BUILTINFS enabled, e.g.:

  undefined reference to `sotest_romfs_img_len'
  undefined reference to `sotest_romfs_img'

Fix by extracting just the bare filename (get_filename_component
... NAME) and running xxd with WORKING_DIRECTORY set to
CMAKE_CURRENT_BINARY_DIR, matching what the Makefile path already
does correctly.

Reported-by: Alan (CI failure on Linux arm64 qemu-armv8a/sotest)
Signed-off-by: aviralgarg05 <gargaviral99@gmail.com>

Author: aviralgarg05

examples/sotest/main/CMakeLists.txt

@@ -91,10 +91,22 @@ if(CONFIG_EXAMPLES_SOTEST)
       DEPENDS ${SOTEST_ROMFS_DEPS}
       VERBATIM)
 
+    get_filename_component(SOTEST_ROMFS_IMG_NAME ${SOTEST_ROMFS_IMG} NAME)
+
+    # xxd -i derives the generated array/length symbol names from the exact path
+    # it is given - sotest_main.c declares them as plain
+    # "sotest_romfs_img"/"sotest_romfs_img_len", so xxd must be run against just
+    # the bare filename (via WORKING_DIRECTORY) rather than the absolute
+    # ${SOTEST_ROMFS_IMG} path, or every non- alphanumeric character in that
+    # path (e.g. every "/" in the build directory) gets folded into the symbol
+    # name instead, leaving the names sotest_main.c expects undefined at link
+    # time.
+
     add_custom_command(
       OUTPUT ${SOTEST_ROMFS_SRC}
-      COMMAND xxd -i ${SOTEST_ROMFS_IMG} > ${SOTEST_ROMFS_SRC}
-      DEPENDS ${SOTEST_ROMFS_IMG})
+      COMMAND xxd -i ${SOTEST_ROMFS_IMG_NAME} > ${SOTEST_ROMFS_SRC}
+      DEPENDS ${SOTEST_ROMFS_IMG}
+      WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 
     set_source_files_properties(${SOTEST_ROMFS_SRC} PROPERTIES GENERATED TRUE)