circleci,modules/coreboot,doc: share crossgcc toolchain across Dasharo 24.12 forks

dasharo_nv4x is the toolchain provider.  dasharo_v56, dasharo_msi_z690,
dasharo_msi_z790 consume its crossgcc via the second coreboot_module()
argument.  All Dasharo boards except novacustom-nv4x_adl are build jobs
(no cache) -- they get crossgcc from workspace and clone fork source fresh.

Fix pre-existing hyphen/underscore variable name typo in modules/coreboot
(lines 179-180 in master) -- the else branch that assigns nobuild overrides
for a shared toolchain module used coreboot-toolchain_module (hyphen)
instead of coreboot_toolchain_module (underscore) matching the variable
defined on line 174.  The bug was latent because no board had ever passed
a non-empty toolchain arg before this PR.  Now that dasharo_v56/msi_z690/
msi_z790 pass dasharo_nv4x as their toolchain, the typo would make the
nobuild override a silent no-op without this fix.

Rename CI jobs for clarity: [cache keys], [ME GBE IFD], [cross compiler],
[seed:coreboot-VERSION].  Update doc/circleci.md to document the cache
model, job types, and Dasharo shared toolchain design.

Signed-off-by: Thierry Laurion <insurgo@riseup.net>

Author: tlaurion