cmake: Allow selecting toolchain file based on config

Refactor IDF "project" functionality under a wrapping of the default
"project" command, so we can tweak it a bit...

Will need more testing in other environments.

components/bootloader/project_include.cmake

@@ -1,4 +1,4 @@
-if(IS_BOOTLOADER_BUILD)
+if(BOOTLOADER_BUILD)
   return()  # don't keep recursing!
 endif()
 

components/bootloader/subproject/CMakeLists.txt

@@ -1,20 +1,17 @@
 cmake_minimum_required(VERSION 3.5)
-set(CMAKE_TOOLCHAIN_FILE $ENV{IDF_PATH}/toolchain.cmake)
 
 if(NOT SDKCONFIG)
   message(FATAL_ERROR "Bootloader subproject expects the SDKCONFIG variable to be passed in by the parent build process.")
 endif()
 
-project(bootloader ASM C CXX)
-
 set(COMPONENTS bootloader esptool_py esp32 soc bootloader_support log spi_flash micro-ecc soc)
 set(BOOTLOADER_BUILD 1)
-set(IS_BOOTLOADER_BUILD 1)  # deprecated, use BOOTLOADER_BUILD
 add_definitions(-DBOOTLOADER_BUILD=1)
 
 set(MAIN_SRCS main/bootloader_start.c)
 
 include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(bootloader)
 
 target_link_libraries(bootloader.elf "-L ${CMAKE_CURRENT_SOURCE_DIR}/main")
 target_link_libraries(bootloader.elf "-T esp32.bootloader.ld")