Documentation updates regarding build

[specing]

Please clarify what is meant under "M451 series SDK" in the README as

• the linked software page contains no package with either "M451" or "SDK" in its name
• the M451 pages do not have any SDK downloads and furthermore the M451 does not seem to have integrated USB (wrong series?)

https://github.com/ClockSelect/myevic/blob/master/git_doc_en/howtobuild_en.md tells us to use some M451_Series_BSP_CMSIS_V3.01.001.zip (I found it at "http://forum.nuvoton.com/viewtopic.php?f=11&t=129" )

After that and replacing all instances of 'arm-none-eabi' with 'arm-eabi' (perhaps provide a PREFIX in make files?), the main build with "EVICSDK=. make " went well. But helloworld's
EVICSDK=/tmp/evic-sdk/ make ends with

[evic-dbg] CC  obj/dbg/evic/main.o
[evic-dbg] LD  bin/dbg/evic/helloworld.elf
/home/specing/opt/GNAT/2018-arm-elf/bin/../lib/gcc/arm-eabi/7.3.1/../../../../arm-eabi/lib/thumb/v7e-m/fpv4-sp/hard//libc.a(lib_a-sbrkr.o): In function `_sbrk_r':
/boron.a/gnatmail/sandbox/community/arm-elf-linux64/newlib_bootstrap-c/build/arm-eabi/thumb/v7e-m/fpv4-sp/hard/newlib/libc/reent/../../../../../../../../../src/newlib/libc/reent/sbrkr.c:58: undefined reference to `sbrk'
make: *** [/tmp/evic-sdk//make/Base.mk:121: bin/dbg/evic/helloworld.elf] Error 1

My toolchain does strange things to C libraries, but sbrk is defined by you anyway and that should be preferred by the build scripts, no? (src/startup/sbrk.c)

[specing]

Theregular Gentoo crossdev arm-none-eabi toolchain fails, as well:

/tmp/evic-sdk-none-eabi/example/helloworld $ EVICSDK=/tmp/evic-sdk-none-eabi/ make                                                                              
[evic-dbg] CC  obj/dbg/evic/main.o
[evic-dbg] LD  bin/dbg/evic/helloworld.elf
/usr/lib/gcc/arm-none-eabi/7.3.0/../../../../arm-none-eabi/lib//libc.a(lib_a-syscalls.o): In function `_sbrk':                                                                  
/tmp/portage/cross-arm-none-eabi/newlib-2.2.0/work/newlib-2.2.0/newlib/libc/sys/arm/syscalls.c:485: undefined reference to `end'                                                
make: *** [/tmp/evic-sdk-none-eabi//make/Base.mk:121: bin/dbg/evic/helloworld.elf] Error 1