Change mynewt package type to sdk

For normal mynewt packages newt tool wants to have specific
folder structure. It wants to have src and include directories,
and only include (and arch related) directory is added to compiler
include search list.
Since TinyUSB has different folder structure newt tool will not
add anything to -I directives and those would need to be
specified as pkg.cflags: "-I@tinyusb/src"

Recent change to newt tool allowed to add specific include directories
for external (sdk) packages so just including package will add
necessary -I to build commands.

This commit changes package type to sdk and specifies src as include
root for TinyUSB.

For older newt tool adding sdk and include_dirs does not break build.

pkg.yml

@@ -5,5 +5,10 @@ pkg.homepage: "https://github.com/hathach/tinyusb"
 pkg.keywords:
     - usb
 
+pkg.type: sdk
+
 pkg.deps:
     - "@apache-mynewt-core/kernel/os"
+
+pkg.include_dirs:
+    - src