cmake: Fix issues when IDF_PATH is not set in environment

Support cases where IDF_PATH may be passed in on the cmake command line,
or inferred from a (hardcoded absolute or relative) path to project.cmake

components/bootloader/subproject/CMakeLists.txt

@@ -5,6 +5,11 @@ if(NOT SDKCONFIG)
         "in by the parent build process.")
 endif()
 
+if(NOT IDF_PATH)
+    message(FATAL_ERROR "Bootloader subproject expects the IDF_PATH variable to be passed "
+        "in by the parent build process.")
+endif()
+
 set(COMPONENTS bootloader esptool_py esp32 soc bootloader_support log spi_flash micro-ecc soc)
 set(BOOTLOADER_BUILD 1)
 add_definitions(-DBOOTLOADER_BUILD=1)
@@ -13,7 +18,7 @@ set(COMPONENT_REQUIRES_COMMON log esp32 soc)
 
 set(MAIN_SRCS main/bootloader_start.c)
 
-include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+include("${IDF_PATH}/tools/cmake/project.cmake")
 project(bootloader)
 
 target_linker_script(bootloader.elf