x86: lift kernel minimum CPU requirement to Pentium MMX

Anything older than that isn't supported since commit f4f8f4a180,
hence also switch to Pentium MMX when building the kernel.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/x86/config-4.9

@@ -264,7 +264,7 @@ CONFIG_KEXEC=y
 CONFIG_KEXEC_CORE=y
 CONFIG_KEYBOARD_ATKBD=y
 # CONFIG_LEDS_CLEVO_MAIL is not set
-CONFIG_M486=y
+CONFIG_M586MMX=y
 # CONFIG_M586 is not set
 # CONFIG_M586MMX is not set
 # CONFIG_M586TSC is not set

target/linux/x86/generic/config-default

@@ -228,7 +228,7 @@ CONFIG_KVM_VFIO=y
 CONFIG_LOCK_SPIN_ON_OWNER=y
 CONFIG_LZO_COMPRESS=y
 CONFIG_LZO_DECOMPRESS=y
-# CONFIG_M486 is not set
+# CONFIG_M586MMX is not set
 # CONFIG_MDA_CONSOLE is not set
 CONFIG_MEMORY_BALLOON=y
 CONFIG_MFD_CORE=y

target/linux/x86/geode/config-default

@@ -79,7 +79,7 @@ CONFIG_ISA_BUS_API=y
 CONFIG_LEDS_GPIO=y
 CONFIG_LZO_COMPRESS=y
 CONFIG_LZO_DECOMPRESS=y
-# CONFIG_M486 is not set
+# CONFIG_M586MMX is not set
 # CONFIG_MDA_CONSOLE is not set
 CONFIG_MFD_CORE=y
 CONFIG_MFD_CS5535=y