build kernel on mac

If we need to build the kernel on mac.
we need some tool to compile the kernel

1. brew install libelf
2. wget https://opensource.apple.com/source/dtrace/dtrace-96/sys/elf.h
3. cp elf.h /usr/local/include/
4. wget https://opensource.apple.com/source/dtrace/dtrace-96/sys/elftypes.h
5. cp elftypes.h /usr/local/include/
6. add the following code into elf.h

#define R_386_NONE 0
#define R_386_32 1
#define R_386_PC32 2
#define R_ARM_NONE 0
#define R_ARM_PC24 1
#define R_ARM_ABS32 2
#define R_MIPS_NONE 0
#define R_MIPS_16 1
#define R_MIPS_32 2
#define R_MIPS_REL32 3
#define R_MIPS_26 4
#define R_MIPS_HI16 5
#define R_MIPS_LO16 6
#define R_IA64_IMM64 0x23 /* symbol + addend, mov imm64 */
#define R_PPC_ADDR32 1 /* 32bit absolute address */
#define R_PPC64_ADDR64 38 /* doubleword64 S + A */
#define R_SH_DIR32 1
#define R_SPARC_64 32 /* Direct 64 bit */
#define R_X86_64_64 1 /* Direct 64 bit */
#define R_390_32 4 /* Direct 32 bit. */
#define R_390_64 22 /* Direct 64 bit. */
#define R_MIPS_64 18

7. brew install gnu-sed --with-default-names
8. export PATH="/usr/local/opt/gnu-sed/libexec/gnubin:$PATH"

git fetch origin rpi-4.14.y:rpi-4.14.y