Squashed commit of the following:

commit 82eb6feacf202bec792aa89f61984ce1ed5c9d53
Author: Huaqi Fang <578567190@qq.com>
Date:   Wed Aug 4 11:01:44 2021 +0800

    SoC: Fix get cpu frequency might be not correct when rtc_clk = cpu_clk

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit ae70519f87d51cb7b5545de0b2fac9a5a5a27995
Author: Huaqi Fang <578567190@qq.com>
Date:   Wed Aug 4 11:00:41 2021 +0800

    build: add SIMU=rtl to choose rtl simulation

    if SIMU=rtl, SIMULATION_RTL will be defined

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 5dcbb442dc1641dbe0f511cffbed317af85f5881
Author: Huaqi Fang <578567190@qq.com>
Date:   Mon Aug 2 19:07:42 2021 +0800

    npk: fix coremark npk build error

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit aa90e22c236a828ee9837f694e198f659ff0a450
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 19:18:21 2021 +0800

    npk: misc npk update for download mode

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 49bdb28c939d61860639392bf0b78e63a166270c
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 19:05:10 2021 +0800

    components: initial folder

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 9bb9878ced8810475c7976910f6dcac5ea3f9a7a
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 19:04:53 2021 +0800

    package: update package version

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 76cbf02dee25a8a2e530a946822c9ccd069b22fa
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 19:04:31 2021 +0800

    misc: Fix ci build for build sdk

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 061901ef45dbfc8009f89c770784b685691358ed
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 19:04:10 2021 +0800

    doc: update README.md

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 19af7b8bc908b5ffa6225b1d6c65560ce9a359d6
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 19:03:48 2021 +0800

    SoC: Add missing build.mk for each SoC

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 21c9e20ce28d5c95b425c8ebfad1a3865928a58b
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 19:03:27 2021 +0800

    doc: update document for build system changes

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit eba00e62d1692630cd49286b20e16cc46348de57
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 17:41:07 2021 +0800

    build: backport nuclei-sdk buildsytem changes

    Easy support for other socs

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 427728f6e9597926cf0fc344acb5b0bf293a5c83
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 17:14:21 2021 +0800

    ci: Fix ci machine tag

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 03c68d4eb5ee614137cb10da3a2f5cb93df6dda8
Author: Huaqi Fang <578567190@qq.com>
Date:   Tue Jul 27 17:12:07 2021 +0800

    npk: update misc fixup in npk

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit b9115030917119188c54b84fc56a638ccc816c01
Author: Huaqi Fang <578567190@qq.com>
Date:   Mon Jul 12 16:29:14 2021 +0800

    npk: Fix demo_plic npk nmsislibsel wrong value

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit c1a79d9bdd42f421943a37aabd9935e1ca1afe83
Author: Huaqi Fang <578567190@qq.com>
Date:   Fri Jul 9 16:25:33 2021 +0800

    npk: initial version of hbird npk files

Signed-off-by: Huaqi Fang <578567190@qq.com>

commit 8195e5b837fb18965d0e5fe3391575d478d3ce4f
Author: Huaqi Fang <578567190@qq.com>
Date:   Fri Jul 9 15:35:32 2021 +0800

    build: add cleanall target for Makefile in SDK root

Signed-off-by: Huaqi Fang <578567190@qq.com>

Signed-off-by: Huaqi Fang <578567190@qq.com>