Port buildroot & keystone sm to hifive unmatched.
Some patches were added from oe-layer, freedom-u-sdk and meta-sifive.
This can generate sdcard image for unmatched by single make command.

Buildroot toolchain is configured to use lp64d,
but opensbi will use lp64 by default.
By adding PLATFORM_RISCV_TOOLCHAIN_DEFAULT=1 to env,
opensbi will use compiler matched abi when compiling.

PLATFORM for opensbi and KEYSTONE_PLATFORM don't always coincide.
KEYSTONE_PLATFORM should be used when specifying sm special sources.

Changes for testing were mixed in.

New files related keystone secureboot are located in src/uboot.
They are copied into extracted u-boot source before building.
patches include only modification in original source.

White early initialization, U-Boot stretch stack to lower addr space
from its load addr `0x80200000`. So the stack will touch pmp protected
space and U-Boot hangs due to access violation.
To avoid this, U-Boot load addr is changed to upper addr, `0x80210000`.

Some files imported from freedom-u-sdk are GPL licensed.
So they're removed from keystone repo and directly downloaded at build-time.