lib: sbi_ecall: Add Kconfig option for each extension

lib: sbi_ecall: Add Kconfig option for each extension

For each SBI extension, we:

- Add a Kconfig option for it
- Add the extension to sbi_ecall_exts only if the extension is enabled
- Add the corresponding sbi_ecall_* object file only if the extension is
  enabled

Special cases are as follows:

- The legacy extensions are lumped together as one 'big' extension, as
  has always been the case in OpenSBI code.
- The platform-defined vendor extensions are regarded as one extension.
- The Base extension cannot be disabled.
- sbi_ecall_replace implements multiple extensions, so it's not easy to
  avoid linking it in. Enable it always, and use #ifdef to
  disable/enable individual extensions.

Signed-off-by: Vivian Wang <dramforever@live.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>