fetch: add SRC_URI checksum

fetch: add SRC_URI checksum

This patch adds per-recipe SRC_URI checksum verification.

- SRC_URI format
The format of SRC_URI checksum follow OE definition:

1. SRC_URI has single src
SRC_URI = "http://some.domain/file.tar.gz"
SRC_URI[md5sum] = "xxxxxxxxxxxxxxx"
SRC_URI[sha256sum] = "xxxxxxxxxxxxxxxxxxxxxx"

2. SRC_URI has multiple src, every src need specify name
SRC_URI = "http://some.domain/file1.tar.gz;name=name1 \
           http://some.domain/file2.tar.gz;name=name2 "
SRC_URI[name1.md5sum] = "xxxxxxxxxxxxxxx"
SRC_URI[name1.sha256sum] = "xxxxxxxxxxxxxxxxxxxxxx"
SRC_URI[name2.md5sum] = "xxxxxxxxxxxxxxx"
SRC_URI[name2.sha256sum] = "xxxxxxxxxxxxxxxxxxxxxx"

- SRC_URI checking invocation:
the checksum checking is invoked in do_fetch phase,
so it can be invoked manually by

if recipes has no SRC_URI checksum item, bitbake will show warning:
"
WARNING: Missing SRC_URI checksum for xxxx.tar.gz, consider to add
SRC_URI[md5sum] = "5c69f16d452b0bb3d44bc3c10556c072"
SRC_URI[sha256sum] = "f4e0ada8d4d516bbb8600a3ee7d9046c9c79e38cd781df9ffc46d8f16acd1768"
"
thus recipe author can add it to recpie file after SRC_URI

- control variable BB_STRICT_CHECKSUM
when SRC_URI checksum is missing, this variable decide pass or not
if BB_STRICT_CHECKSUM = "1", missing checksum is fatal

Signed-off-by: Yu Ke <ke.yu@intel.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>
Signed-off-by: Chris Larson <chris_larson@mentor.com>