[RFC v3 2/2] Add initial RISC-V architecture support

[Andrey Korolyov]

> +CONFIG_TEXT_BASE=0x80000000
> +CONFIG_MEMORY_LAYOUT_FIXED=y
> +CONFIG_STACK_BASE=0x807f0000
> +CONFIG_STACK_SIZE=0x10000
> +CONFIG_MALLOC_BASE=0x80800000

Due to changed offsets in e300/u500 configurations (as compared to v2-
qemu config) linkage fails with binutils-2.25.1 and gcc-5.3.0:

arch/riscv/boot/built-in.o: In function `main_entry':
/build/barebox/arch/riscv/boot/main_entry.c:34:(.text+0x0): relocation
truncated to fit: R_RISCV_HI20 against symbol `__bss_start' defined in
.bss section in barebox
Makefile:756: recipe for target 'barebox' failed