kbuild: rpm-pkg: Fix manual debuginfo generation when using .src.rpm

kbuild: rpm-pkg: Fix manual debuginfo generation when using .src.rpm

Commit 62089b804895 ("kbuild: rpm-pkg: Generate debuginfo package
manually") added uses of OBJCOPY and READELF, variables from Kbuild.
These variables are defined and work properly when using the binrpm-pkg
target because rpmbuild is run within Kbuild. However, these variables
are not defined when building from a source RPM package generated with
the srcrpm-pkg target, breaking the build when generating the debug info
subpackage.

Define a default value for these variables so that these commands
respect the value from Kbuild but continue to work when built from a
source RPM package.

Cc: stable@vger.kernel.org
Fixes: 62089b804895 ("kbuild: rpm-pkg: Generate debuginfo package manually")
Reported-by: Lukas Herbolt <lukas@herbolt.com>
Closes: https://lore.kernel.org/20260212135855.147906-2-lukas@herbolt.com/
Tested-by: Lukas Herbolt <lukas@herbolt.com>
Link: https://patch.msgid.link/20260213-fix-debuginfo-srcrpm-pkg-v1-1-45cd0c0501b9@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>