[U-Boot] U-Boot SPL/TPL build system

[Kever Yang]

Hi Simon,Philipp,

     I still confuse about how the SPL/TPL build system works, including 
what Kconfig will be used,
and how to write the Makefile suppose to be correct.

     I'm trying to enable the SPL/TPL for rk3328, I want to remove all 
the unnecessary module because the
ram size is very limited, but the instruction size in aarch64 is lager.
     For example,
- for debug output, I only want to enable the DEBUG_UART, but not 
SERIAL_DM and SERAIL_SUPPORT.
     If disable the SPL_SERIAL_DM and SERIAL_SUPPORT, it can not pass 
the build.
- I don't need interrupt, which vector table need a lot size in aarch64,
- I don't need panic, this realy dose not help me much, I don't want to 
enable sysreset in TPL;


For the rk3368, I get overflow when I use aarch64 compile tool from 
ubuntu 14.04:
aarch64-linux-gnu-ld.bfd: u-boot-tpl section `.u_boot_list' will not fit 
in region `.sram'
aarch64-linux-gnu-ld.bfd: region `.sram' overflowed by 224 bytes
If I use the gcc-linaro-6.3.1-2017.05-x86_64_aarch64-linux-gnu, it won't 
overflow.

Thanks,
- Kever