Move applying savedconfig from src_configure() into kernel-build_merge_configs(), in order to make it possible to override
saved config options. Previously, the saved config would
unconditionally overwrite everything, which could be between
inconvenient and plain broken (particularly if savedconfig contained
signing key paths referring to ${T}).

The new logic applies saved config via merge method, on top
of the default config along with ebuild and eclass overrides. However, MODULES_SIGN_KEY* and user config snippets do override saved config
for convenience.

Closes: https://bugs.gentoo.org/938130
Signed-off-by: Michał Górny <mgorny@gentoo.org>
---
eclass/kernel-build.eclass | 30 +++++++++++++++++++++---------
1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/eclass/kernel-build.eclass b/eclass/kernel-build.eclass
index c4f3db0028a9..1eac9433b65c 100644
--- a/eclass/kernel-build.eclass
+++ b/eclass/kernel-build.eclass
@@ -167,8 +167,8 @@ kernel-build_pkg_setup() {

# @FUNCTION: kernel-build_src_configure
# @DESCRIPTION:
-# Prepare the toolchain for building the kernel, get the default .config
-# or restore savedconfig, and get build tree configured for modprep.
+# Prepare the toolchain for building the kernel, get the .config file,
+# and get build tree configured for modprep.
kernel-build_src_configure() {
debug-print-function ${FUNCNAME} "${@}"

@@ -243,7 +243,6 @@ kernel-build_src_configure() {
MAKEARGS+=( KBZIP2="lbzip2" )
fi

- restore_config .config
[[ -f .config ]] || die "Ebuild error: please copy default config into .config"

if [[ -z "${KV_LOCALVERSION}" ]]; then
@@ -594,11 +593,15 @@