libopkg: store checksums in binary form, use integer index for architecture

libopkg: store checksums in binary form, use integer index for architecture

Instead of storing a copy of the architecture string and architecture
priority value in each pkg_t instance, declare a 3 bit wide field which
allows referencing the architecture in the global config array by index.

The 3 bit field allows referencing up to 8 different architectures which
is more than enough for the systems we target with opkg. Another nice side
effect is that we can coalesce this field with other flag values in pkg_t,
saving 4 bytes for an int member.

Also convert the hexadecimal checksums to binary format before storing them in
pkg_t's blob buffer to save 50% of the space per checksum.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>