Throw an error if androidboot.hardware is used for bootconfig

Bootconfig does not allow the keys to have values and subkeys at the
same time.
"androidboot.hardware" and "androidboot.hardware.*" subkeys have existed as
kernel cmdline parameters. We want to throw an error at build time when
"androidboot.hardware" is use in bootconfig because it may be a common
occurence while migrating to bootconfig.
"hardware" should be used instead, without the "androidboot" prefix.

Test: build with and without "androidboot.hardware=test" in
BOARD_BOOTCONFIG
Bug: 173815685

Change-Id: Ic62a86cb23c9d79dbdbacd18eed3449210566ce7