trees: avoid kconfig make commands generically

don't hardcode the check based on whether the current
project is grub. instead, define "btype" in target.cfg

if unset, we assume kconfig and permit kconfig commands
e.g. make menuconfig, make silentoldconfig, etc

this is to avoid the deadliest of sins:
project-specific hacks

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/grub/default/target.cfg

@@ -3,3 +3,4 @@ rev="8719cc2040368d43ab2de0b6e1b850b2c9cfc5b7"
 bootstrapargs="--gnulib-srcdir=gnulib/ --no-git"
 autoconfargs="--with-platform=coreboot --disable-werror"
 makeargs="FS_PAYLOAD_MODULES=\"\""
+btype="autohell"