[eclass] sabayon-kernel: drop old SRC_URI setup code
[sys-kernel/sabayon-sources] drop unused K_SABKERNEL_URI_CONFIG
[eclass] sabayon-kernel: drop K_SABKERNEL_URI_CONFIG
[sys-kernel/efikamx*] drop ancient kernel, no longer supported
[eclass] sabayon-kernel: simplify kernel config search code