Let user specify existing PREFIX/RISCV for toolchain installs

[jerryz123]

I like to manage my $RISCV independently.

[alonamid]

In this case, if you've installed riscv-tools, and now decide you want to install esp-tools, you need to actively clean your RISC-V environment variable (otherwise it will silently now install esp-tools). Is there a way to indicate this to the unsuspecting user?

[jerryz123]

In this case, if you've installed riscv-tools, and now decide you want to install esp-tools, you need to actively clean your RISC-V environment variable (otherwise it will silently now install esp-tools). Is there a way to indicate this to the unsuspecting user?

Why would I need to actively clean my RISCV directory? The script should just overwrite existing files in there when it builds esp-tools.

[a0u]

What @alonamid probably meant was, if one intends to build an alternative toolchain but has already sourced env.sh, the user must remember to unset RISCV to avoid overwriting the original installation.

I am fine with supporting arbitrary installation paths, but an explicit command line option (-d <path>) would be preferable instead. Regardless of the approach, the user-specified input should also be converted into an absolute path, if not already one, to avoid a subsequent configure error.

[sagark]

+1 to @a0u

[jerryz123]

I added a --prefix option. I also incremented the CI cached tools index, so the build-tools tests reran.

[alonamid]

LGTM