[BCM SAI] update BCM SAI to 4.2.1.5 (sonic-net#5866)

This release includes changes for supporting Debian Buster, fixes for internally found issues and more enhancement related to spec coverage and feature parity for Broadcom ASICs.

Additional fixes included:

CS00011465498 - Warm reboot
CS00011465061 - interfaces not coming up
CS00011396506 - nexthop resource leak
CS00011452080 - BCM SAI crash while getting lane count

platform/broadcom/sai.mk

@@ -1,8 +1,8 @@
-BRCM_SAI = libsaibcm_4.2.1.3-1_amd64.deb
-$(BRCM_SAI)_URL = "https://sonicstorage.blob.core.windows.net/packages/bcmsai/4.2/master/libsaibcm_4.2.1.3-1_amd64.deb?sv=2015-04-05&sr=b&sig=1%2BrMfgxeSc%2BXJUPw6X%2FhNGxe4KUp15Oqq1lVwMJGiW0%3D&se=2034-07-20T03%3A20%3A08Z&sp=r"
-BRCM_SAI_DEV = libsaibcm-dev_4.2.1.3-1_amd64.deb
+BRCM_SAI = libsaibcm_4.2.1.5-4_amd64.deb
+$(BRCM_SAI)_URL = "https://sonicstorage.blob.core.windows.net/packages/bcmsai/4.2/master/libsaibcm_4.2.1.5-4_amd64.deb?sv=2015-04-05&sr=b&sig=dHfQf95FxgLBlnrGJwv6pIkAYAc6mOGbz9vvUMfxFvg%3D&se=2034-07-28T05%3A53%3A21Z&sp=r"
+BRCM_SAI_DEV = libsaibcm-dev_4.2.1.5-4_amd64.deb
 $(eval $(call add_derived_package,$(BRCM_SAI),$(BRCM_SAI_DEV)))
-$(BRCM_SAI_DEV)_URL = "https://sonicstorage.blob.core.windows.net/packages/bcmsai/4.2/master/libsaibcm-dev_4.2.1.3-1_amd64.deb?sv=2015-04-05&sr=b&sig=lTzaCynflt5UO7Ltf2Z%2B6zHJ4R%2BFj421kupIGO4%2FOCI%3D&se=2034-07-20T03%3A19%3A27Z&sp=r"
+$(BRCM_SAI_DEV)_URL = "https://sonicstorage.blob.core.windows.net/packages/bcmsai/4.2/master/libsaibcm-dev_4.2.1.5-4_amd64.deb?sv=2015-04-05&sr=b&sig=n98j842RZy%2BwQh4CtOku7jz6wgV%2BusbTl9NkeRndBrE%3D&se=2037-01-13T05%3A54%3A34Z&sp=r"
 
 SONIC_ONLINE_DEBS += $(BRCM_SAI)
 $(BRCM_SAI_DEV)_DEPENDS += $(BRCM_SAI)