/xtensa-lx106-elf/bin/../lib/gcc/xtensa-lx106-elf/8.4.0/../../../../xtensa-lx106-elf/lib/crt0.o:(.literal+0x0): undefined reference to `main' collect2: error: ld returned 1 exit status make[1]: *** [/Users/xu756/Downloads/hello_world/build/bootloader/bootloader.elf] Error 1 make: *** [/Users/xu756/Downloads/hello_world/build/bootloader/bootloader.bin] Error 2

/xtensa-lx106-elf/bin/../lib/gcc/xtensa-lx106-elf/8.4.0/../../../../xtensa-lx106-elf/lib/crt0.o:(.literal+0x0): undefined reference to `main' collect2: error: ld returned 1 exit status make[1]: *** [/Users/xu756/Downloads/hello_world/build/bootloader/bootloader.elf] Error 1 make: *** [/Users/xu756/Downloads/hello_world/build/bootloader/bootloader.bin] Error 2 #1747