config: Add common config for book-style foldables

Book-style foldables like the Pixel Fold, Galaxy Z Fold and Xiaomi MIX
Fold series have two displays: one primary, near tablet-sized inner
panel and a smaller, phone-sized outer panel.

These devices need a combination of configs to enable both phone- and
tablet-targeted features. Therefore, add a config file merging contents
of the common phone and tablet configs.

Change-Id: Ibf89e4336fa119ebfb79aad8dac4266fd4972985
Signed-off-by: Jens Reidel <adrian@travitia.xyz>