bootimg: exynos: extract platform and subtype from dt.img header

dtbtool-exynos has two options --platform and --subtype that are
embedded in dt.img header and need to match the values that the
bootloader expects. For most devices these values are 0x50a6 and
0x217584da, respectively, but for some they have other values.

Add functionality to parse the dt.img header and extract these values,
and add them to the deviceinfo as
bootimg_qcdt_exynos_{platform,subtype} if they are not equal to the
default values.

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2638

pmb/types.py

@@ -135,6 +135,8 @@ class Bootimg(TypedDict):
     cmdline: str
     bootimg_qcdt: str
     bootimg_qcdt_type: str | None
+    bootimg_qcdt_exynos_platform: str | None
+    bootimg_qcdt_exynos_subtype: str | None
     dtb_offset: str | None
     dtb_second: str
     base: str